Changed severity to "feature".
The message is just telling, that a translation file for a language was added that can't be set in user preferences.

@atrol Thank you very much for the deatails. :-)

I just checked on master branch, and the only occurrence of this message for en-gb is in the built-in Gravatar plugin.

Translations to British English are disabled on TranslateWiki for MantisBT (including bundled plugins), so this language file should not be there. I'll remove it.

With regards to similar warnings for other languages, (e.g. Indonesian/id, Kara-Kalpak/kaa, Bengali/bn), etc. that appear in some bundled plugins but not under MantisBT core, the reason is that TranslateWiki only sends us the language files when they reach a certain translation threshold (currently 25%), and we only add them to the configuration after receiving them; this is a manual process.

These warnings can be safely ignored.