What is it like to know two languages?

To all multilingual ones here, I pose this question: what is it like knowing multiple languages? Do you think in different languages, change when the environment does, or switch between them randomly? Do you ever get your words mixed up (i.e., you're talking to an English speaker but instead of saying cheese you say queso)? Do you find that knowing another language helps or hinders your understanding of your native one?

Pick and choose whatever you want to answer, but thank you for your time either way!

I am German and consider myself fluent in English and Spanish. I'm studying Japanese too, but I'm still far from fluent (I can read like 500 kanji or so, you need around 2 or 3 thousand to read a novel).

I mostly think (and talk to myself...) in English, since I pretty much live on the Internet and read all my books and tutorials in this language. Ever since I started surrounding myself more and more with Japanese though, some Japanese words and sentence structures started popping up, like "you're my slave, dakara!". But I can keep this under control and it doesn't happen to me when I talk to people.
However, my Japanese is pretty much overwriting my Spanish, which I don't use in any meaningful way anymore. Maybe it's because I used to watch anime with Spanish subtitles, or because they use similar sounds/phonemes, I dunno. As of now, I can hardly formulate two sentences in Spanish without using Japanese words. "gomen, señor..."
I never mix up spoken German and English. I do sometimes use English spellings of Latin/Greek loanwords in German, however.
I usually sound things out in my head when I read something. Japanese/Chinese writing however emphasizes meaning over sound, and I frequently forget how to read certain words even though I remember what the characters mean. The first thing that pops into my mind in that situation is usually an English word (sometimes a German one) that I tend to sound out as Engrish. For example I might read 虫の音が聞こえる静かな夜 as "mushi no ne ga kikoeru shizuka na nighto"

>Do you find that knowing another language helps or hinders your understanding of your native one?
As for input, it definitely helps me understand my native language. And not just the language itself, but also the concepts that are expressed in it.
However, as far as output goes, I think it's more useful to be really good at just one or two languages than to be mediocre at several. I might move to Canada one day... in that case, I'll probably forget most of my German and my already deteriorating Spanish through disuse.