That's a good point. I suppose that the quality of the translation could also be a factor. If for example I only slightly prefer my native language over English, the website could (theoretically) spare me the poorly machine translated page in my native language over the original page in English. Multiply the client's quality assessment of the relative degradation in quality given language options with the server's quality assessment of the localizations on offer and serve the content with the highest score.

Fun anecdote: I remember seeing a screenshot of a website selling a scented candle called "basic bitch", which was machine translated so that the idiomatic meaning was entirely lost. So the translation ended up meaning something like "foundational female dog".