For instance on Wikipedia, where interface doesn't follow content language: https://translatewiki.net/wiki/File:Wikipedia_zh-tw_view.png
And on TWN before login (with zh.tw in preferences): https://translatewiki.net/wiki/File:Translate.net_zh-tw_view.png
I hope I filed this bug correctly, some background is on https://translatewiki.net/wiki/Thread:Talk:Main_Page/Translate.net_Main_page_does_not_show_right_translation_for_zh-tw_users_by_default (and there are several more related Support threads in the last years, IIRC).
This is probably a duplicate but I couldn't find the correct bug: bug 31838 is unrelated, bug 12521 I don't know and anyway both bug 13463 and bug 8056 (similar) should be fixed.
Version: unspecified
Severity: normal