I tried to save the file in txt format (just using the Save as command in Notepad), but it didn't work.
Caroline, you have to Save as *Trados TXT* in *Olifant*! It's not the same as saving as TXT in Notepad!
Both TMX and Trados TXT are text files anyway, but not in the same format. I think Trados is more picky with TMX than with Trados TXT, which is why I suggested the latter.
I will see if I can correct the language code, but the question I had was what Unicode format shall I save it as? UTF-8?
If your language pair is EN-FR, I don't think it really matters whether the TM is Unicode or not. I would use whatever Olifant suggests, and change it only if you see strange characters (or question marks instead of special characters) in Trados. But having the right language codes (ie. the codes expected by Trados) is important!