I have a tendency to change interests frequently too. Which is why I'm forcing myself to stick with things.

For example, I go through phases where I collect stuff like sometimes LEGO sometimes Pokemon stuff and I can't seem to make my mind up on what kind of stuff I want to collect.

I also go through hobby phases, I'll be all interested in learning a language then it will change and I'll lose interest.

Keeping yourself interested is hard, especially with hobbies such as language learning. There are going to be dry parts that just don't interest you at all. Pick something, or a few things and try to stick with them for a year.

Right now I am learning Italian, Japanese, violin. I keep myself interested by setting goals for myself. Ex: I would like to get good enough in Italian that I can read the Italian edition of Harry Potter that I bought. I would like to play violin good enough to where I can play songs as good as Lindsey Stirling. These are big goals but they help keep me motivated.