I had the thought that maybe if they are similar it might make it more difficult. For me, it seems like know if 2 languages that are quite different would be an easier switch in the brain. I only speak English so I have no idea what I am talking about, though. 

Yeah, most definitely the similar languages overlap but this doesn't mean you can't get lost in the specifics...

ch0k1

alter_native

Learning a new language makes it harder to remember previously learned languages

I wonder if it could depend on the languages. Since some are more similar. Interesting. 