One example: Most documentation/resources available online are in English, so I use most applications in English, despite that not being my native language so t

One example: Most documentation/resources available online are in English, so I use most applications in English, despite that not being my native language so that I'll have it easier to find support online. There might be other apps where I prefer them to be in my native language (e.g. news apps) or where the use of legal/financial terms makes it difficult as a non-native speaker (e.g. banking/trading apps). Lastly, it might be interesting for language learners or people regularly switching between languages or countries. But you might be right that all of those use cases only affect a too small user group, so that there simply is no incentive for developers to add a language switching feature.