I have a finnish admin-version (Versio 3.8.0+180522). I just copied an multilingual existing survey and wanted to make some modification in that. When copying, it took only the main language (finnish). Then I wanted to "manually" add english, german and swedish, but they are not listed in the language list. The swedish language were found when I typed it into the box, but english and german not. The german informal language is listed.
I went to config pages to change the admin language and saw under language tab, that for some reason 5 languages were hidden, among them Swedish, German and English. So everything is perfect now!