Maxolino I cannot really imagine a use case where I don't write the prompt in the language I want the response to be in
I'm sure that's true for you personally as one single individual. 😃
Any biligual people working as part of an organisation - be it an employer or a FOSS community - that prefer to use their own native language but require generated code comments / email drafts / other output from The Assistant be in the common language of the organisation (typically, though not always, this will be English) might reasonably ask what the point of having a language setting in the UI is if it's ignored in favour of matching the prompt.
Indeed, I would argue that those using a second (or third...) language they aren't as comfortable in would be more likely - not equally or less likely - than others to lean on such AI tools to help them in their written communications. While Kagi Translate also exists, it sort of assumes you already have an appropriate message to provide as the input - and I don't see why whole groups of people should be expected - rather unnecessarily, in my view - to spend more time using multiple tools when it seems that a single tool should reasonably be expected to be sufficient.
Then there's additional use cases - such as someone simply wanting every conceivable opportunity to practice using a language they are actively trying to learn, even when everyday contacts don't use that language themselves.
Going back to the bilingual example from earlier - one could invert it and ask about examples of, say, a web developer who is searching for help writing some JavaScript and all of the results (e.g. Stack Overflow) will inevitably be in English - but just because the input they copy and paste is in English, doesn't mean that's the language they would prefer to receive a response in.
While some or all of these quick examples might seem daft to you, there's an entire world out there of different languages and cultures that sees people in different circumstances relying upon tools such as Kagi Translate, The Assistant, Kagi Search, and more besides for a whole host of different use cases. Each of those users is valid, and each of their perspectives deserves to be respected.