This is mainly a bug in the implementation of mkv support in the player.
The mkv standard specifies that undefined should be mapped to English.
Too be frank this is partially also the fault of mkvtoolnix since older versions of it didn't use the undefined for English but the 'eng/en' language flag. Newer versions of mkvtoolnix don't support the 'eng/en' language flag even if it's specified by the command line,... (which is kind of sad, but that's the way mosu likes it)
-> see mkvtoolnix thread for additional info