You are correct, accept-language simply doesn't work in practice for too many users, so everyone uses IP based detection. It is very common to have an interface

You are correct, accept-language simply doesn't work in practice for too many users, so everyone uses IP based detection. It is very common to have an interface language not being a preferred one or even not the one people understand well enough to read text beyond some common interface words, which is especially common for English, for example.