I think when two languages are too similar to each other (but not the same), it can cause problems when learning. I tried learning Portuguese after learning Spanish in HS. Big mistake. The two languages are so similar that I end up mixing them up when writing.
There were also situations when I thought I knew how to say something in Portuguese (because that's how it's said in Spanish and Portuguese has similar words), but it turned out that that phrasing isn't used at all.
Didn't think of it like that, I can imagine that it was a problem mixing those two languages up!