I do speak Vietnamese at somewhat of a conversational level, and yes I did mention it in another thread over in Young Adults

I worked sooo hard to do it because of the language barrier.
Books.

Lots of them. I had a crazy amount of vietnamese/english dictionaries, I had all the "learn the language" books, I had the Book+CD sets so I could learn to pronounce it properly, I even downloaded software to help me learn it.
Chinese is hard to learn to write due to the character set, where as I found vietnamese slightly easier in writing because it uses many letters similar to the english alphabet. But I don't know if Chinese is different than Vietnamese in verbally learning it because Vietnamese is what's known as a tonal language.. you can have the vietnamese word "Ma" with 6 different kinds of accents over the letter A and it makes that one word mean 6 different things based on your intonation of the vowel! It was -insane-

You could quite literally set out to say one thing, and completely say another thing that had no connection with your original intention
I don't know if Chinese is the same. If it's tonal, that's a whole added difficulty!
I do know a good amount of Japanese, which is not a tonal language and shares a character set with Chinese.. I still can't -write- that character set well at all though, haha.
But yeah.. books. I also let my ex husband's mother try to teach me basic lessons on occasion, but because she hated me so much (nice to my face, talked about me "behind my back" [sometimes while I was standing right there, which was my motivation behind -learning- the language oddly enough]) it was really difficult to spend any time with her. If you enjoy your SO's family, ask them for some lessons. My ex-MIL didn't know a ton of english either but, when teaching the -basics- of her language, it didn't really matter (hold up an apple, tell me what it is in her language.. point to a table, tell me what it is.. you can learn a lot of basics that way. Plus she taught me the alphabet, and basic sentence structure, and so on)