I've been learning Vietnamese for about three years now. In many ways it's a fairly easy language to learn. The grammar is very simple and it uses the Latin alphabet.

But learning to hear and speak the various tones has been a real struggle. Even now I still have a hard time picking them out and carefully enunciating them. It doesn't help that there's a huge amount of regional variation in pronunciation and vocabulary. I've had to work hard to unlearn some habits from English, particularly raising my tone slightly at the end of sentences.