On language forums, people ask all the time something to the effect of, “I just learned Spanish– can I learn Italian without mixing up words?” Or a variety of other combinations: Spanish/Portuguese, Spanish/French, French/Italian, German/Swedish and so on and so on.
While I do think it’s most efficient to leave “space” between similar languages (like, maybe learn a different language in between Spanish and Portuguese…. Or just wait a longer period of time before you start), mixing up words really isn’t the end of the world.
It helps to take bilingual children as an example. While the linguistic acquisition principles aren’t exactly the same, you’ll find that children raised from birth with more than one language (for example, the one parent, one language thing) mix languages all the time. I spoke to a 3 year old Italian-American kid who said something to the effect of “I have pane!” Kids do this, especially in their younger years, because the two languages develop at different paces, and they haven’t yet figured out how to separate the vocabulary of one from the vocabulary of the other. So, they want to express something, and they just logically insert the word from the other language. It actually makes sense!
I was a little worried when I started learning Spanish after first learning French. I like to practice languages by thinking in them– kind of narrating whatever I’m doing in the whatever language I’m studying. But I kept finding over and over that the French word would jump to mind immediately, making it difficult to wrack my brains and get the Spanish word out. I thought it might be because the languages were so similar– and perhaps that had something to do with it (similar grammars allow you to more easily insert related words!), but that wasn’t the whole story.
French, my first “second” language, was much better than my Spanish, and for several years, it was my only “second” language. So, I suspected that whenever I wanted to extract a “second” language word from my mind, my brain was like, “Here, French! That’s what you wanted, right?” It was a matter of habit plus a much bigger French vocabulary.
Now that my Spanish vocabulary is pretty sizeable, I don’t encounter the same problem nearly as often. I still have problems switching between languages– if I’m reading, listening, or thinking in French, suddenly switching to Spanish is like stopping a moving train. It takes a while to get the Spanish vocabulary going. But it’s very comforting to know that the problem resolved itself with simply advancing in Spanish.
I’m still wary of very similar languages. I’m really interested in Portuguese, but my Spanish feels too recently acquired, and I don’t want to mess it up. And Spanish is way more important to me at the moment. Portuguese is very, very close to Spanish, so, I might wait on that one. I’m interested in Yiddish, but given that I’m currently working on Dutch, I’m not sure if I’d mess Dutch up for the sake of Yiddish. On the other hand, Dutch isn’t exactly instrumental in my day-to-day life, so maybe that one’s worth the experiment 🙂
In conclusion, in my experience, mixing up languages is not the death knoll you think it is. It’s a temporary problem, but you hardly have to focus on it for it to resolve itself. Good luck!