Topics in default language are always listed

Expected behavior: forum should only display topics in the current language
What happened instead: the default lanuages's topics are always shown beside language-specific topics.
Steps to reproduce the bug (in this example, domain-based language negotiation is used):
1. Set up a multilanguage site with en the default and de the other allowed language. Configure forum vocabulary to have per-language terms.
2. Set up terms in the forum vocabulary for each language.
3. Visit en.example.org/forum - only the english forum topics are displayed. Visit de.example.org/forum. Both English and German forum topics are shown.
4. Switch the default language to de. de.example.org/forum now lists only German forum topics, but en.example.org/forum displays both English and German forum topics.