I think it's a valid configuration to have different languages in different browsers. For instance your laptop could have English as your working language, and then you could prefer Spanish on another device you use for more personal things. In addition, the browsers could have been built with different languages, and require langpacks to support them. This would mean ensuring that langpacks are supported for the browser, and can be downloaded.
I think my recommendation here would not to support it unless we find new clear evidence that it should be. This is for two reasons:
- The user could prefer to have multilingual devices.
- There is complexity and risk in supporting it due to multiple configuration and builds, and the requirement of downloading and installing langpacks.
If we did decide to support this feature, then I think it would need some additional considerations such as adding it to the list of options to sync, since a user might prefer not to actually sync a language. I would consider this a new feature request, and not a bug.