I'm not familiar with the language pack you're using, but it looks like they don't have strings for FOF Links. You could use Flagrow Linguist to add them on your site (although the real irony here might be the Linguist, too, will show up like this).
The long term solution would be to send your language pack or FOF Links the translations for their strings.
As for why a fallback isn't used, this issue slightly explains it. It's definitely an area Flarum can improve on, and just hasn't had the chance yet.
