Thanks Rytis, Very helpful. At one time I was trying to learn Spanish and French as a beginner, too similar, too confusing. I had already thought to wait until Russian is clearer in my mind before thinking about learning another language. You have confirmed that, or at least for me and the way I learn.

Mixing up languages is due to one or more of them being at a low level. It's not because they are at different levels. So some of what you wrote is incorrect, and some of your advice ineffective. 

Agree with the fellow above about mixing up languages being a problem of incomplete learning / language acquistion.  I spent give years on Korean (for travelling and survival conversation -- it's easy to learn to read and write that script) and three years on Japanese (academic focus -- to pass an exam) and now (10 years later) -- I can recall words / phrases in each language but I often have to think hard about what language the word is.  I have to remember how it's spelled (hangul??? or hiragana/katakana???) to tell them apart and that's because I was only a low intermediate speaker in each.  The things that I know well (the basics of conversation, some nouns, verbs, place names) -- these I 'know' and don't mix up.

Your point about persona / taking on the second language identity really struck home for me though because I never was able to make the necessary shift to identifying with Korean / Japanese culture enough to make self-expression feel normal and natural.  It always felt like I was 'outside' the culture and that the psychic boundary was ... pretty tough to transcend (though it was always pleasurable to study these very foreign languages, esp the kanji / hanja -- so insightful about how and where East and West perceive the world...)

Well, we also have to remember this is the guy who said that politics never has any place in language learning.  Completely ridiculous, so let's consider the source.