I assume (hope) you have used the feature for multi-lingual questionnaires. -- Yes. Compliments on the design! It has been quite easy to use.
Usually no completion check (invalid or missing answers are marked red) should be performed when switching the language. -- You are right. It must have been some other problem that I was having. Now I checked (tried filling it out myself in the pre-test) and switched languages, and the language code switches automagically within the collected data set.
Really helpful! Have a nice day :)