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.
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...
If its a more passive thing (such as pondering something whilst actually working on something else), or my language 'skills' for lack of a better word are doing something else (e.g. listening to music with lyrics), then the thoughts are more abstract ideas and images, and the monologue only returns when I come to some sort of conclusion. It's interesting to note that this happens more frequently when I've been using languages other than English.
Brains definitely are weird.
This isn't common at all, but the belief that it's happening is common.
What's actually happening is that people's thoughts automatically trigger language production, and they hear the words, the output of that process, in their head. But the thought comes first and the mental words are a side effect. Imagine your brain having an internal narrator who constantly reads your thoughts and shouts them out as formed sentences.
I can formulate sentences in my mind, but it feels really slow and cumbersome.
But what really blew my mind was when I realized that people think with words. Its always "have an idea, then attempt to serialize it into some approximation in the form of linear sentence or words". I mean, I can chose to speak a sentence in my head, but it is strange and artificial. But from what I hear from others, apparently people actually think about things with words, and which words they have both enable and limit their thinking. I still don't exactly know how that works, but apparently its common?