Mixing up languages
I have had a whole lot of exposure to Japanese. Thousands of hours of anime, living in a location with lots of native Japanese speakers for a few years, and two years of learning it in the classroom. To me Japanese is very natural and I have no problem thinking in it, and that's probably for around 1500-2000 words.
I'm trying to produce sentences in Spanish without translating to English mentally, but when I do that my sentences come out with a mix of Spanish and Japanese words or a mix of Spanish and Japanese grammar or even both, usually with Japanese being more dominant. This has recently become a much bigger problem because I watched a few episodes of anime for the first time in a while and it just triggered my Japanese knowledge or something.
How do you keep things straight when you aren't just translating to English first? I feel like I'll never be able to smoothly speak/write/think in Spanish because of this.
Try creating different personas for each language you study. For example, I try to act and sound more macho when I speak and think in Russian and I act more humble and polite when I speak Japanese. Also make sure your accent is very distinct for each language. I also use the method of loci for remembering words and store vocabularies of each language in different places in my mind. For example I store my French words in my elementary school and my Spanish in my High school. It is very rarely I mix up words and I study over 10 languages.
To think in another language is a different ball game than simply being fluent in the language. At the minimum, you have to immerse in the language, Spanish in this case, to the level you did in Japanese. That said, one can be fluent in more than 2 languages and do quite well. So you will mix few words now and then but it should not be to the point that you won't be able to communicate efficiently and effectively. Personally, I have more trouble with mixing related languages but occasionally I mix non related languages. Not to sound controversial, from my experience in communicating with bi,trilingual speakers I noticed that, in general, the females, my wife included but not only, tend to mix more the languages they know but they keep going, that is, they don't seem to get bothered by it to the extent that you seem to. In case it is not clear, I am talking about people that are "fluent" in more than one language. This tends to happen when talking about topics that are not fully mastered in the language spoken but which are well known in a second or third language. Personally, I get frustrated in situations like that and usually I realize the lack of knowledge but occasionally I do mix two languages when the other party knows them too! So, it maybe that females don't make a fuss about it as long as they know they get the message across?? The bottom line is that you are not alone so keep improving your Spanish! HTH, Daniel.
The gender observation may not mean anything or it may. Your case could very well be the exception to the rule rather than the rule! Anyway, it was a side point for which I have no empirical data other than my own undocumented impressions... The important points were that there could be reasons for when one behaves the way you mentioned and that you were not alone! Maybe I should qualify my original statements: I am no linguist let alone a scholar in the domain. I am simply an amateur interested in languages who happens to speak more than one language on a daily basis...
I think it'll work itself out with time. I suppose I'll always remember a random Portuguese word popped out unbidden as I was trying to say something in Russian class, but such things just happen. 2nd languages seem to get grouped in a certain part of the brain leaving words from them more immediately accessible when there's a gap in your knowledge (or flow of thought) in a language you have less competence in. So you get Japanese in your Spanish. If you know no Japanese, it would just be English filling in the gaps in the Spanish, so I don't think you're any worse off at least!