It looks like a twin of No longer remember the last input directory? (http://avidemux.org/smif/index.php/topic,17106.0.html): a language selected via Edit --> Preferences --> Language doesn't make it to the config file. Hence the GUI uses again the language provided by the environment upon restart.
Fixed by [i18n] Fix language selection (https://github.com/mean00/avidemux2/commit/0126c4fa391dd6271e896a52fd2d7537969c87cd#diff-339579312774b85d7f37dedd6a8bd2b5R594), thanks!