support the PostHog app in languages other than English

[christiaan-ph]

Is your feature request related to a problem?

The PostHog UI is only available in English. Users who prefer another language fall back on browser translation (Chrome's built-in translation or extensions), which modifies the page DOM and frequently crashes the app with errors like NotFoundError: Failed to execute 'removeChild' on 'Node'. PostHog detects this and shows a banner asking the user to turn off the translation extension, which leaves non-English speakers with no good option: use the app in a language they don't read, or have it break.

Describe the solution you'd like

Native localization of the PostHog app UI, so the product can be used in other languages without relying on browser translation.

Describe alternatives you've considered

• Browser page translation: causes React DOM crashes (Bug: Error in React's DOM cleanup process #46669, changing timeframe of dashboard breaking while using google translate #19663), and the app's error boundary explicitly warns against it
• Making the app resilient to DOM-modifying translators: would reduce crashes but still leaves users dependent on machine translation quality

Additional context

• The error boundary warning shown to affected users: frontend/src/layout/ErrorBoundary/ErrorBoundary.tsx
• Related but distinct: Feature request: Internationalization and localization for surveys, workflows, etc. #42154 covers internationalization of user-created content (surveys, workflows, early access descriptions). This request is about the PostHog product UI itself.
• There is currently no i18n framework in the frontend; all UI strings are hardcoded in English
• Adjacent internationalization work exists, such as IME input handling (fix(ui): ignore Enter key during IME composition #51350)
• This issue can serve as the tracking point for customer requests for a translated UI

Debug info

• PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
• PostHog Hobby self-hosted with docker compose, version/commit: [please provide]
• PostHog self-hosted with Kubernetes (deprecated, see Sunsetting Kubernetes support), version/commit: [please provide]