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'm a French native speaker and I can use english (especially technical english).
I have a small hability to think in english, and thanks to that I don't have to deal with a translation phase when I want to speak or write.
I learn German some years, but me level is very bad :'(

I put often english words in my french sentence, but it's computer science related word (such as kernel, OS, ...).
2 days ago, a strange things happen to me : I though a french phrase with the world 'underlying'. At this moment I tried to search to french world but I don't remember it. It took 10 minutes to me to remember of the word ('sous-jacent').

Well, since I'm fluent in reading and listening english, I like learning tongues. Soon I'll start 日本語.