Every now and then, I'll forget a word in English but remember it in another language. My internal monologue tends to be English-ish (especially right now, when I'm composing a post), but if the sounds for a word are missing, the concept will fill its place...or sometimes the closest equivalent of whichever language I've been thinking about lately. I can also think almost completely in wordless concept-strings, but that sometimes feels forced.
Maybe I should mention that I learned a few words in French, German, and Spanish before I was 8, but the first time that I really studied a language in the usual sense was when I was about 12.
Do you ever get frustrated trying to explain this? I've had people refuse to believe me, and I end up feeling like I'm not thinking "as well" or "as rigorously" as I might be.
I have a feeling that forgetting words isn't particularly unusual even for native speakers though.
After I began to think in English (my second language), I found that I wasn't speaking my first language anymore, I was just translating to it from my second one. And because the two weren't connected in my brain, I started frequently forgeting words and having trouble with phrasing.
Mildly off-topic, but I never realised until recently how much of a difference this can make. I've used French numbers far more, and for far longer, than I have Japanese numbers [0], but learning that way still results in me going "8 - that's eight, so its huit". Because I learnt the Japanese numbers through usage and without the deliberate English comparison, I go straight to "8 - hachi" without the intermediate step.
It's also weird to me that I can count backwards (say 10 to 1) far easier in Japanese than in French, probably for similar reasons.
[0] Note I'm a long way from fluent in either. I studied French at school, and have picked up a very small amount of Japanese through usage.
What is quite scary is that my colleagues now understand from tone and context exactly which "thingy" I am referring to...
The simplest proof I have for the fact that I don't think in words is that I quite often forget a word, and can't complete a sentence. I know exactly what I want to express and have thought it through in my mind, but I can't tell it to someone else because I don't remember the serialization of that idea, and am forced to try to work around it by describing the word and hoping someone fills it in for me.