I know what you mean. Coming from a Python background I first wrote Javascript in a Python-ish way. Now it's the other way around and I put semicolons at the end of the line, parentheses around logic clauses and use the wrong logic operators in my Python code. This because I work on both code bases simultaneously (Javascript on the client, Python on the server)...
I have used regularly Python for backend and JavaScript for frontend at the same time (right now I am mainly using Python), and I never got those kind of cross-language problems...