[Bug] Locale fr-be (Belgian French) incorrectly served as Belarusian (be) UI

[Wallon007]

Description

The DeepSeek web interface (chat.deepseek.com) is served in Belarusian to users whose browser sends fr-be (Belgian French) in the Accept-Language header, even when fr and fr-fr are also present and ranked higher.

This appears to be a confusion between the ISO 3166-1 alpha-2 country code BE (Belgium) and the ISO 639-1 language code be (Belarusian).

Steps to reproduce

1. Use Firefox (tested on version 155) with the following Accept-Language preference list for websites:
   • fr-be (Français Belgique)
   • fr (Français)
   • fr-fr (Français France)
   • en-us (English US)
2. Connect from a Belgian IP address (country: BE).
3. Navigate to https://chat.deepseek.com.

Expected behavior

The interface should be displayed in French, since fr-be, fr, and fr-fr are all present in Accept-Language and all resolve to the French language.

Actual behavior

The interface is displayed in Belarusian, despite no Belarusian locale being requested and despite the user being located in Belgium (a country where French, Dutch, and German are official languages — but not Belarusian).

Workaround

Removing fr-be from the preferred languages list in Firefox (keeping only fr and fr-fr) resolves the issue immediately. This strongly suggests the backend is matching on the substring be inside fr-be and treating it as the Belarusian language code.

Impact

• Affects all French-speaking Belgian users.
• Cannot be resolved client-side without removing the legitimate fr-be locale, which impacts date/number formatting on other sites.
• This is a standards-compliance issue: Accept-Language should be parsed per RFC 4647 / BCP 47, and regional subtags (BE) must not be confused with primary language subtags (be).

Suggested fix

Parse Accept-Language using a proper BCP 47 parser, and match on the primary language subtag (fr in fr-be) rather than performing substring matching on the full tag. Region subtags (BE, FR, CA, etc.) should never be interpreted as language codes.

Environment

• Browser: Firefox 155
• OS: (Linux Debian or Fedora)
• IP geolocation: Belgium (BE)
• Date: 2026-09-10

Thank you for looking into this.