panic: message "simplified_explanation" not found in language "th" on missing translation keys #2070

Describe the bug

When a client sends a request with an unhandled/partially supported language in the Accept-Language header (e.g., th-TH,th;q=0.9), Anubis panics when serving the challenge page due to missing translation keys in the go-i18n localizer. This results in a 502 Bad Gateway error for the client.

Instead of falling back gracefully to the default language (e.g., en), calling MustLocalize panics and crashes the request handler.

Steps to reproduce

1. Run the Anubis Docker container with default settings.

2. Send an HTTP request to the Anubis-protected endpoint with the HTTP header Accept-Language: th-TH,th;q=0.9 (or any locale where simplified_explanation is missing).

3. Observe the challenge page being triggered (issueChallenge).

4. The server panics with panic: message "simplified_explanation" not found in language "th" and returns a 502 Bad Gateway.

Expected behavior

When a requested language key or locale is missing, Anubis should fall back to the default language (English) or use Localize with proper error handling/fallback instead of calling MustLocalize which triggers a runtime panic.

Your operating system and its version.

iPhone OS 18_7 like Mac OS X

Your browser and its version.

AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.6.1 Mobile/15E148 Safari/604.1

Additional context

Log Output / Stack Trace
http: panic serving 172.18.0.6:48896: message "simplified_explanation" not found in language "th" goroutine 4963 [running]: net/http.(*conn).serve.func1() net/http/server.go:1939 +0xac panic({0x2529b18?, 0x7f62f72db0?}) runtime/panic.go:859 +0x120 github.com/nicksnyder/go-i18n/v2/i18n.(*Localizer).MustLocalize(0x2893388?, 0x7f62794270?) github.com/nicksnyder/go-i18n/v2@v2.6.1/i18n/localizer.go:235 +0x3c github.com/TecharoHQ/anubis/lib/localization.(*SimpleLocalizer).T(...) github.com/TecharoHQ/anubis/lib/localization/localization.go:121 ...