feat(i18n): enable String Catalog + locale-aware numeric formatters

Lays the groundwork for zh-Hans / de / fr translations on an English base.
No user-visible English-locale behavior changes. See scarf/docs/I18N.md for
the full plan and remaining audit follow-ups.

- Localizable.xcstrings seeded with 538 keys auto-extracted via
  `xcstringstool sync` from the Swift sources
- InfoPlist.xcstrings carrying NSMicrophoneUsageDescription
- knownRegions += zh-Hans, de, fr
- Currency / byte-count / compact-number String(format:) sites migrated to
  Locale.current-aware .formatted() style (currency, byteCount(.file),
  compactName notation) — previously rendered POSIX separators + English
  unit names regardless of user locale

Refs #13.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/docs/I18N.md

@@ -0,0 +1,107 @@
+# Internationalization (i18n)
+
+Scarf uses Apple's modern **String Catalog** workflow. Source strings are auto-extracted from `Text("…")` and `String(localized: …)` literals into [`scarf/scarf/Localizable.xcstrings`](../scarf/Localizable.xcstrings) at build time (when built in Xcode.app; `xcodebuild` alone emits per-source `.stringsdata` but does not merge back into the catalog). Info.plist keys are localized via [`scarf/scarf/InfoPlist.xcstrings`](../scarf/InfoPlist.xcstrings).
+
+## Languages
+
+| Locale | Status |
+|---|---|
+| `en` (English) | Base / source |
+| `zh-Hans` (Simplified Chinese) | Region enabled, translations pending |
+| `de` (German) | Region enabled, translations pending |
+| `fr` (French) | Region enabled, translations pending |
+
+Canadian French users are served by base `fr`. `fr-CA` will be added only if a concrete Québec-specific bug is reported.
+
+## Adding a new language
+
+1. Xcode → Project → Info → Localizations → `+` (add locale).
+2. Ensure the locale code is also listed in `knownRegions` of `scarf.xcodeproj/project.pbxproj`.
+3. Open `Localizable.xcstrings` in Xcode; the new locale appears as an empty column — translate or use Xcode's AI suggestions.
+4. Repeat for `InfoPlist.xcstrings` (microphone usage, etc.).
+5. Smoke-test via scheme language override (Edit Scheme → Run → App Language).
+
+## Adding translations (AI-first workflow)
+
+For the three supported non-English locales we use Xcode's built-in AI translation:
+
+1. Open `Localizable.xcstrings` in Xcode.
+2. Select untranslated rows for a locale → right-click → **Translate** (Xcode 26+ provides GPT-backed suggestions with context from the surrounding code comment).
+3. Review each suggestion before marking **Translated**.
+4. For terms that should NOT translate (proper nouns like *Scarf*, *Hermes*, *Anthropic*; env var names; file paths), wrap the source site in `Text(verbatim: "…")` so the key never hits the catalog.
+
+## Guardrails when writing new UI code
+
+`Text("literal")` auto-localizes. These patterns **silently leak English** and need explicit handling:
+
+| Pattern | Fix |
+|---|---|
+| `Text(someStringVar)` | `Text(LocalizedStringResource("key"))` or pass a `LocalizedStringKey` down the view tree |
+| `"Hello " + name` | `String(localized: "Hello \(name)")` |
+| `String(format: "$%.2f", cost)` | `cost.formatted(.currency(code: "USD").precision(.fractionLength(2)))` |
+| `String(format: "%.1f MB", size)` | `Int64(size).formatted(.byteCount(style: .file))` |
+| `String(format: "%.1fM", n)` | `n.formatted(.number.notation(.compactName))` |
+| Custom `DateFormatter` with fixed `dateFormat` | `date.formatted(.dateTime.month().day().year())` |
+| `.help(stringVar)` | Compute a `LocalizedStringKey` or use `.help(Text(…))` |
+| `Button(stringVar)` | `Button(LocalizedStringResource("key")) { … }` |
+
+Strings that are **user data** (session titles, memory file contents, log lines, shell commands shown in UI, file paths) should pass through without localization — this happens naturally when the value is a `String` variable, since those overloads skip the catalog.
+
+## Outstanding audit follow-ups
+
+The [initial i18n PR](#) enabled the catalog infrastructure and migrated clear locale-bug formatter sites. The following patterns remain in the codebase and should be refactored **before** translations ship in Phase 2 — otherwise their English copy will leak through regardless of locale:
+
+### Category A — UI copy held in a `String` variable (needs `LocalizedStringResource`)
+
+High-priority (ternary UI copy, visible status chrome):
+
+- `Features/Health/Views/HealthView.swift:135` — `"Hermes Running"` / `"Hermes Stopped"` ternary
+- `Features/Chat/Views/ChatView.swift:125` — `"Active"` fallback
+- `Features/Chat/Views/ChatView.swift:241` — `"Voice On" / "Voice Off"`
+- `Features/Chat/Views/ChatView.swift:256` — `"TTS On" / "TTS Off"`
+- `Features/Chat/Views/ChatView.swift:271` — `"Recording..." / "Push to Talk"`
+- `Features/MCPServers/Views/MCPServerTestResultView.swift:13` — `"Test passed" / "Test failed"`
+- `Features/Profiles/Views/ProfilesView.swift:145` — `"Active profile" / "Inactive"`
+- `Features/Platforms/Views/PlatformSetup/SignalSetupView.swift:54` — `"signal-cli is available on PATH"` / not-found message
+- `Features/QuickCommands/Views/QuickCommandsView.swift:148` — `"Add Quick Command"` / `"Edit /\(name)"` ternary
+- `Features/MCPServers/Views/MCPServersView.swift:131` — `.help("\(count) tools")` vs `"Test failed"` ternary
+- `Features/MCPServers/Views/MCPServerDetailView.swift:157, 185` — masked-value placeholder
+
+Medium-priority (enum `.rawValue` → display):
+
+- `Features/Logs/Views/LogsView.swift:30,38,48,69` — file / component / level display
+- `Features/Projects/Views/ProjectsView.swift:153` — tab display
+- `Features/Skills/Views/SkillsView.swift:37` — tab display
+- `Features/Insights/Views/InsightsView.swift:40, 201` — period picker, day-of-week names (see also Category C)
+- `Features/CredentialPools/Views/CredentialPoolsView.swift:265` — type display
+- `Features/Activity/Views/ActivityView.swift:117` — kind display
+
+Lower-priority (displayName passthroughs from services that could be wrapped once at the source):
+
+- `Features/Platforms/Views/PlatformsView.swift:43, 91`, `Features/Tools/Views/ToolsView.swift:46` — `platform.displayName`
+- `Features/Gateway/Views/GatewayView.swift:105` — `platform.name.capitalized`
+- `Features/MCPServers/Views/*` — `transport.displayName`, preset names / descriptions
+- `Features/Servers/Views/ServerSwitcherToolbar.swift:40`, `ManageServersView.swift:96` — `server.displayName`
+
+### Category B — Composite format strings with translatable suffixes
+
+- `Features/Settings/Views/Components/ModelPickerSheet.swift:105` — `ctx + " ctx"` — literal " ctx" needs keying
+- `Features/Insights/Views/InsightsView.swift:93` — `formatTokens(...) + " tokens"` — literal " tokens" needs keying
+- `Features/MCPServers/Views/MCPServerTestResultView.swift:15` — `"%.1fs · %d tools"` — needs splitting into a `String(localized: "\(elapsed)s · \(count) tools")`
+- `Features/Insights/Views/InsightsView.swift:167` — `"%.1f%%"` — needs `.formatted(.percent)` after verifying the input range (0…1 vs 0…100)
+
+### Category C — Hard-coded day / month name arrays
+
+- `Features/Insights/Views/InsightsView.swift:196` — `["Mon", "Tue", …]` literal — use `Calendar.current.shortWeekdaySymbols` (locale-aware).
+
+### Category D — `.help(stringVar)` sites
+
+- `Features/Servers/Views/ConnectionStatusPill.swift:42` — `.help(tooltip)`; refactor `tooltip` to return `LocalizedStringKey` / `LocalizedStringResource`.
+
+### Non-blocking (intentional verbatim)
+
+The following are correct as-is because they pass user data or machine-readable content through to the UI:
+
+- Session titles, message content, memory / skill / YAML file contents, log lines, shell commands, file paths, session IDs, model IDs, credential sources, URL strings.
+
+If we later need to badge these (e.g. "(empty)" placeholder), the badge itself becomes a localizable key while the data passthrough stays verbatim.

scarf/scarf.xcodeproj/project.pbxproj

@@ -214,6 +214,9 @@
 			knownRegions = (
 				en,
 				Base,
+				"zh-Hans",
+				de,
+				fr,
 			);
 			mainGroup = 534959372F7B83B600BD31AD;
 			minimizedProjectReferenceProxies = 1;

scarf/scarf/Core/Models/ProjectDashboard.swift

@@ -86,8 +86,8 @@ enum WidgetValue: Codable, Sendable, Hashable {
         case .string(let s): return s
         case .number(let n):
             return n.truncatingRemainder(dividingBy: 1) == 0
-                ? String(Int(n))
-                : String(format: "%.1f", n)
+                ? Int(n).formatted(.number)
+                : n.formatted(.number.precision(.fractionLength(1)))
         }
     }
 

scarf/scarf/Core/Services/ModelCatalogService.swift

@@ -20,7 +20,8 @@ struct HermesModelInfo: Sendable, Identifiable, Hashable {
     /// Display-friendly cost string, or nil if cost is unknown.
     var costDisplay: String? {
         guard let input = costInput, let output = costOutput else { return nil }
-        return String(format: "$%.2f / $%.2f", input, output)
+        let currency = FloatingPointFormatStyle<Double>.Currency.currency(code: "USD").precision(.fractionLength(2))
+        return "\(input.formatted(currency)) / \(output.formatted(currency))"
     }
 
     /// Display-friendly context window ("200K", "1M", etc.).

scarf/scarf/Features/Chat/Views/SessionInfoBar.swift

@@ -45,7 +45,8 @@ struct SessionInfoBar: View {
                 }
 
                 if let cost = session.displayCostUSD {
-                    Label(String(format: "$%.4f%@", cost, session.costIsActual ? "" : " est."), systemImage: "dollarsign.circle")
+                    let formattedCost = cost.formatted(.currency(code: "USD").precision(.fractionLength(4)))
+                    Label(session.costIsActual ? formattedCost : "\(formattedCost) est.", systemImage: "dollarsign.circle")
                         .contentTransition(.numericText())
                 }
 
@@ -75,11 +76,6 @@ struct SessionInfoBar: View {
     }
 
     private func formatTokens(_ count: Int) -> String {
-        if count >= 1_000_000 {
-            return String(format: "%.1fM", Double(count) / 1_000_000)
-        } else if count >= 1_000 {
-            return String(format: "%.1fK", Double(count) / 1_000)
-        }
-        return "\(count)"
+        count.formatted(.number.notation(.compactName).precision(.fractionLength(0...1)))
     }
 }

scarf/scarf/Features/Dashboard/Views/DashboardView.swift

@@ -114,7 +114,7 @@ struct DashboardView: View {
                 StatCard(label: "Tokens", value: formatTokens(viewModel.stats.totalInputTokens + viewModel.stats.totalOutputTokens))
                 let cost = viewModel.stats.totalActualCostUSD > 0 ? viewModel.stats.totalActualCostUSD : viewModel.stats.totalCostUSD
                 if cost > 0 {
-                    StatCard(label: "Cost", value: String(format: "$%.2f", cost))
+                    StatCard(label: "Cost", value: cost.formatted(.currency(code: "USD").precision(.fractionLength(2))))
                 }
             }
         }
@@ -217,7 +217,7 @@ struct SessionRow: View {
                 Label("\(session.messageCount)", systemImage: "bubble.left")
                 Label("\(session.toolCallCount)", systemImage: "wrench")
                 if let cost = session.displayCostUSD, cost > 0 {
-                    Label(String(format: "$%.4f", cost), systemImage: "dollarsign.circle")
+                    Label(cost.formatted(.currency(code: "USD").precision(.fractionLength(4))), systemImage: "dollarsign.circle")
                 }
             }
             .font(.caption)

scarf/scarf/Features/Insights/Views/InsightsView.swift

@@ -61,10 +61,10 @@ struct InsightsView: View {
                 InsightCard(label: "Cache Write", value: formatTokens(viewModel.totalCacheWriteTokens))
                 InsightCard(label: "Reasoning Tokens", value: formatTokens(viewModel.totalReasoningTokens))
                 InsightCard(label: "Total Tokens", value: formatTokens(viewModel.totalTokens))
-                InsightCard(label: "Total Cost", value: String(format: "$%.2f", viewModel.totalCost))
+                InsightCard(label: "Total Cost", value: viewModel.totalCost.formatted(.currency(code: "USD").precision(.fractionLength(2))))
                 InsightCard(label: "Active Time", value: formatDuration(viewModel.activeTime))
                 InsightCard(label: "Avg Session", value: formatDuration(viewModel.avgSessionDuration))
-                InsightCard(label: "Avg Msgs/Session", value: viewModel.sessions.isEmpty ? "0" : String(format: "%.1f", Double(viewModel.totalMessages) / Double(viewModel.sessions.count)))
+                InsightCard(label: "Avg Msgs/Session", value: viewModel.sessions.isEmpty ? "0" : (Double(viewModel.totalMessages) / Double(viewModel.sessions.count)).formatted(.number.precision(.fractionLength(1))))
             }
         }
     }

scarf/scarf/Features/Sessions/ViewModels/SessionsViewModel.swift

@@ -159,12 +159,7 @@ final class SessionsViewModel {
         let dbPath = context.paths.stateDB
         let fileSize: String
         if let stat = context.makeTransport().stat(dbPath) {
-            let size = Double(stat.size)
-            if size >= FileSizeUnit.megabyte {
-                fileSize = String(format: "%.1f MB", size / FileSizeUnit.megabyte)
-            } else {
-                fileSize = String(format: "%.0f KB", size / FileSizeUnit.kilobyte)
-            }
+            fileSize = Int64(stat.size).formatted(.byteCount(style: .file))
         } else {
             fileSize = "unknown"
         }

scarf/scarf/Features/Sessions/Views/SessionDetailView.swift

@@ -60,7 +60,8 @@ struct SessionDetailView: View {
                     Label("\(session.reasoningTokens) reasoning", systemImage: "brain")
                 }
                 if let cost = session.displayCostUSD {
-                    Label(String(format: "$%.4f%@", cost, session.costIsActual ? "" : " est."), systemImage: "dollarsign.circle")
+                    let formattedCost = cost.formatted(.currency(code: "USD").precision(.fractionLength(4)))
+                    Label(session.costIsActual ? formattedCost : "\(formattedCost) est.", systemImage: "dollarsign.circle")
                 }
                 if let date = session.startedAt {
                     Label(date.formatted(.dateTime.month().day().hour().minute()), systemImage: "calendar")

scarf/scarf/Features/Settings/Views/Components/SettingsComponents.swift

@@ -224,7 +224,7 @@ struct DoubleStepperRow: View {
                 .font(.caption)
                 .foregroundStyle(.secondary)
                 .frame(width: 160, alignment: .trailing)
-            Text(String(format: "%.2f", value))
+            Text(value.formatted(.number.precision(.fractionLength(2))))
                 .font(.system(.caption, design: .monospaced))
                 .frame(width: 70, alignment: .leading)
             Stepper("", value: Binding(

scarf/scarf/InfoPlist.xcstrings

@@ -0,0 +1,18 @@
+{
+  "sourceLanguage" : "en",
+  "strings" : {
+    "NSMicrophoneUsageDescription" : {
+      "comment" : "Shown by macOS when Scarf first requests microphone access for Hermes voice chat.",
+      "extractionState" : "manual",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Scarf uses the microphone for Hermes voice chat."
+          }
+        }
+      }
+    }
+  },
+  "version" : "1.0"
+}