I've been learning Welsh for the past few years - our class in our English town Skypes to our teacher in Wales, and it all comes up on the Smart board in the classroom. There are occasionally technical difficulties, but usually it works really well.
There's some good online stuff. Duolingo is good for speaking, but doesn't explain anything about the grammar or anything. Say something in Welsh has been mentioned by a few people.
I've learnt French, Spanish, German and Latin before, (no non-European languages, so can't compare with Mandarin) and Welsh has definitely been the hardest to learn. It took me ages to get my head round mutations - some words mutate at the start of the word, and it can feel like there are about a billion rules. When you're first learning, it can make looking up unknown vocabulary somewhat challenging. But also, you realise on speaking, it won't matter as much, because people will still understand, so I just got more relaxed about not always getting it right.
Also, there are differences between north and south Wales - the word for milk is different, for example, and there are some differences in grammatical construction's, which aren't a big deal after a while, but are confusing when you're starting.
My main issue though, is that I find it harder to remember vocabulary than I did when I was younger. Every week, I'll be reading something and think, I've seen that word before, and I just cannot remember what it is. Or I'm trying to say something, and I can think of the word in Spanish or German or French, but not Welsh. This is me, not Welsh, though!
But I still enjoy it. Don't care if I never speak it for real, but I like being able to read it.