Any dialogues popped up by FLTK (e.g. fl_choice()), will always have the title "ThinLinc Client", no matter the configured language.
This is because we set the title before we've loaded the translations. As far as I can tell, this bug has been here since we started setting the titles in bug 3755.
Also note bug 7937, which might make this bug irrelevant.
A common example of one of these dialogs is the host key dialogs.