diff --git a/src/renderer/src/components/github-project/ProjectRoadmap.plugin-locale.test.tsx b/src/renderer/src/components/github-project/ProjectRoadmap.plugin-locale.test.tsx
new file mode 100644
index 000000000000..3e3b092a1391
--- /dev/null
+++ b/src/renderer/src/components/github-project/ProjectRoadmap.plugin-locale.test.tsx
@@ -0,0 +1,106 @@
+// @vitest-environment happy-dom
+
+import type { ReactNode } from 'react'
+import { cleanup, render, screen } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import ProjectRoadmap from './ProjectRoadmap'
+import { i18n, setRendererPluginLanguagePacks } from '@/i18n/i18n'
+import { DEFAULT_LOCALE } from '@/i18n/supported-languages'
+import { pluginLanguageResourceId } from '../../../../shared/plugins/plugin-language-pack-artifact'
+import type { GitHubProjectRow, GitHubProjectTable } from '../../../../shared/github/project-types'
+
+vi.mock('@/components/ui/tooltip', () => ({
+  Tooltip: ({ children }: { children: ReactNode }) => <>{children}</>,
+  TooltipTrigger: ({ children }: { children: ReactNode }) => <>{children}</>,
+  TooltipContent: ({ children }: { children: ReactNode }) => <div role="tooltip">{children}</div>
+}))
+
+// The exact pack from the field crash report (v1.4.219, Windows).
+const PACK_ID = 'plugin:stablyai.orca-portuguese/pt-BR' as const
+const PACK_RESOURCE = pluginLanguageResourceId(PACK_ID)
+
+function roadmapTable(): GitHubProjectTable {
+  const row: GitHubProjectRow = {
+    id: 'one',
+    itemType: 'ISSUE',
+    content: {
+      number: 7,
+      title: 'Scheduled',
+      body: null,
+      url: 'https://github.com/o/r/issues/7',
+      state: 'OPEN',
+      stateReason: null,
+      isDraft: null,
+      repository: 'o/r',
+      assignees: [],
+      labels: [],
+      parentIssue: null,
+      issueType: null
+    },
+    fieldValuesByFieldId: {
+      f_start: { kind: 'date', fieldId: 'f_start', date: '2026-09-01' },
+      f_end: { kind: 'date', fieldId: 'f_end', date: '2026-10-15' }
+    },
+    updatedAt: '2026-08-31T00:00:00Z',
+    position: 0
+  }
+  return {
+    project: {
+      id: 'PVT_1',
+      owner: 'stablyai',
+      ownerType: 'organization',
+      number: 3,
+      title: 'Orca',
+      url: 'https://github.com/orgs/stablyai/projects/3'
+    },
+    selectedView: {
+      id: 'PVTV_1',
+      number: 2,
+      name: 'Roadmap',
+      layout: 'ROADMAP_LAYOUT',
+      filter: '',
+      fields: [
+        { kind: 'field', id: 'f_title', name: 'Title', dataType: 'TITLE' },
+        { kind: 'field', id: 'f_start', name: 'Start date', dataType: 'DATE' },
+        { kind: 'field', id: 'f_end', name: 'Target date', dataType: 'DATE' }
+      ],
+      groupByFields: [],
+      sortByFields: []
+    },
+    rows: [row],
+    totalCount: 1,
+    parentFieldDropped: false
+  }
+}
+
+afterEach(async () => {
+  cleanup()
+  window.localStorage.clear()
+  setRendererPluginLanguagePacks([])
+  await i18n.changeLanguage(DEFAULT_LOCALE)
+})
+
+describe('ProjectRoadmap with a plugin language pack active', () => {
+  it('formats ticks with the pack locale instead of the synthetic resource tag', async () => {
+    setRendererPluginLanguagePacks([
+      {
+        id: PACK_ID,
+        resourceLanguage: PACK_RESOURCE,
+        pluginKey: 'stablyai.orca-portuguese',
+        locale: 'pt-BR',
+        // Non-empty so i18next resolves to the pack, as a shipped pack does.
+        catalog: { common: { cancel: 'Cancelar' } }
+      }
+    ])
+    await i18n.changeLanguage(PACK_RESOURCE)
+    expect(i18n.resolvedLanguage).toBe(PACK_RESOURCE)
+
+    render(<ProjectRoadmap table={roadmapTable()} fallback={<div>list</div>} />)
+
+    const expectedMonth = new Intl.DateTimeFormat('pt-BR', {
+      month: 'short',
+      timeZone: 'UTC'
+    }).format(new Date(Date.UTC(2026, 8, 1)))
+    expect(screen.getAllByText(expectedMonth, { exact: false }).length).toBeGreaterThan(0)
+  })
+})
diff --git a/src/renderer/src/components/github-project/ProjectRoadmap.tsx b/src/renderer/src/components/github-project/ProjectRoadmap.tsx
index 5e06e41d8700..26c0c40824ee 100644
--- a/src/renderer/src/components/github-project/ProjectRoadmap.tsx
+++ b/src/renderer/src/components/github-project/ProjectRoadmap.tsx
@@ -3,7 +3,7 @@ import { CalendarClock } from 'lucide-react'
 import { Button } from '@/components/ui/button'
 import { usePrefersReducedMotion } from '@/hooks/usePrefersReducedMotion'
 import { cn } from '@/lib/utils'
-import { i18n, translate } from '@/i18n/i18n'
+import { getIntlLocale, translate } from '@/i18n/i18n'
 import ProjectGroupHeader from './ProjectGroupHeader'
 import ProjectRoadmapBar from './ProjectRoadmapBar'
 import { ProjectTitleCell } from './ProjectCellIdentity'
@@ -48,7 +48,7 @@ export default function ProjectRoadmap({
 }: Props): React.JSX.Element {
   const view = table.selectedView
   const prefersReducedMotion = usePrefersReducedMotion()
-  const locale = i18n.resolvedLanguage ?? i18n.language
+  const locale = getIntlLocale()
   // Why: the grid lives on UTC calendar days (parseRoadmapDate), so "today"
   // must be the viewer's LOCAL calendar date mapped to UTC midnight — the raw
   // instant would shift the marker into the wrong day off UTC.
diff --git a/src/renderer/src/i18n/no-raw-language-intl-locale.test.ts b/src/renderer/src/i18n/no-raw-language-intl-locale.test.ts
new file mode 100644
index 000000000000..563d9a05b2ae
--- /dev/null
+++ b/src/renderer/src/i18n/no-raw-language-intl-locale.test.ts
@@ -0,0 +1,91 @@
+/**
+ * Plugin language packs make `i18n.language` a synthetic `plugin<hex>` tag that
+ * every `Intl` constructor rejects with a RangeError, so Intl locales must come
+ * from `getIntlLocale()`. This catches the two shapes that shipped that crash:
+ * the raw language passed straight to Intl, or held in a `locale` variable.
+ */
+import { readdirSync, readFileSync, statSync } from 'node:fs'
+import { relative, resolve } from 'node:path'
+// TypeScript 7 is a native CLI; AST tests still need the legacy JavaScript API.
+import ts from 'typescript-api'
+import { describe, expect, it } from 'vitest'
+
+const RENDERER_ROOT = resolve('src/renderer/src')
+const LOCALE_METHODS = new Set([
+  'toLocaleString',
+  'toLocaleDateString',
+  'toLocaleTimeString',
+  'localeCompare'
+])
+const RAW_LANGUAGE = /\b(?:i18n|i18next)\??\.(?:language|resolvedLanguage)\b/
+
+function collectSourceFiles(dir: string, files: string[] = []): string[] {
+  for (const name of readdirSync(dir)) {
+    const filePath = resolve(dir, name)
+    if (statSync(filePath).isDirectory()) {
+      collectSourceFiles(filePath, files)
+    } else if (
+      /\.(ts|tsx)$/.test(name) &&
+      !/\.test\.(ts|tsx)$/.test(name) &&
+      !filePath.includes('/i18n/locales/')
+    ) {
+      files.push(filePath)
+    }
+  }
+  return files
+}
+
+function isLocaleSink(node: ts.CallExpression | ts.NewExpression, file: ts.SourceFile): boolean {
+  const callee = node.expression
+  if (callee.getText(file).startsWith('Intl.')) {
+    return true
+  }
+  return ts.isPropertyAccessExpression(callee) && LOCALE_METHODS.has(callee.name.text)
+}
+
+describe('Intl locale source', () => {
+  it('never feeds the raw i18n language to Intl', () => {
+    const violations: string[] = []
+
+    for (const filePath of collectSourceFiles(RENDERER_ROOT)) {
+      const source = readFileSync(filePath, 'utf8')
+      if (!RAW_LANGUAGE.test(source)) {
+        continue
+      }
+      const file = ts.createSourceFile(
+        filePath,
+        source,
+        ts.ScriptTarget.Latest,
+        true,
+        filePath.endsWith('.tsx') ? ts.ScriptKind.TSX : ts.ScriptKind.TS
+      )
+      const report = (node: ts.Node): void => {
+        const { line } = file.getLineAndCharacterOfPosition(node.getStart(file))
+        violations.push(`${relative(process.cwd(), filePath)}:${line + 1}`)
+      }
+
+      function visit(node: ts.Node): void {
+        if (
+          (ts.isCallExpression(node) || ts.isNewExpression(node)) &&
+          isLocaleSink(node, file) &&
+          node.arguments?.some((arg) => RAW_LANGUAGE.test(arg.getText(file)))
+        ) {
+          report(node)
+        } else if (
+          ts.isVariableDeclaration(node) &&
+          ts.isIdentifier(node.name) &&
+          /^locale$/i.test(node.name.text) &&
+          node.initializer &&
+          RAW_LANGUAGE.test(node.initializer.getText(file))
+        ) {
+          report(node)
+        }
+        ts.forEachChild(node, visit)
+      }
+
+      visit(file)
+    }
+
+    expect(violations).toEqual([])
+  }, 15_000)
+})