[lolinder]

There are layers to thought, and the layer that is most conscious is, at least for me (and from what I hear a lot of other people), in my native language. Further, achieving fluency in a second language is often associated with skipping the intermediate step of translating from English, with thoughts materializing first in the second language.

You're correct that there does seem to be a layer lower than that—one that can materialize as either a native or a second language—but it's not inaccurate to talk about which language we "think" in because many of us actually do constantly materialize thoughts as language without any intention of speaking them.

[fsckboy]

you can talk to yourself in your head using your native language. But that's not evidence that you are thinking in your language, it's thinking of your language. when you do math, are you thinking in your language? when you drive a car or play a video game or a sport, or make love, are you thinking in your language? I'll answer for you, no, you aren't.

Do people who grow up without language (there have been plenty examples, deaf people for example) simply not think? do cats and dogs and chimpanzees not think?

[lolinder]

I don't think we actually disagree on the facts so much as on the buckets we sort them into.

I very much count my internal English monologue as thinking in my language and would understand that to be what someone is asking about if they asked what language I think in. At the same time, I totally agree that there are layers to thought that never rise into the monologue.

[juliendorra]

Thinking is probably not a single unified process, but several simultaneous activities at different levels. Including the unconscious ones. So you might very well both be right, as we may be thinking sometimes simultaneously in language and in other ways (like when we talk and cook, when we write, etc.)

[drdeca]

There are kinds of thought which are facilitated by language.