When you edit wikitext by using certain languages, the interface is in bold (all of it except the editing field itself). There is various <strong> tags on the HTML code, out or in defined divs.
- problem in Breton: https://fr.wikipedia.org/w/index.php?title=Utilisateur:Trizek_(WMF)&action=edit&uselang=br
- no problem in French: https://fr.wikipedia.org/w/index.php?title=Utilisateur:Trizek_(WMF)&action=edit&uselang=fr
I haven't found a problem with an other language yet.
That's apparently not related to the skin, because I have that bug on Monobook too: