How to change App Store language without changing region?

It can't be done. You have to use the App Store designated for the region in which your account is located. You don't have any control over the language that App Store uses.

A tiny few countries have App Stores in more than one language. An example is Canada, which has both an English language and a French language App Store.

M1 Mac mini, macOS 26.2;  Watch, Series 10 GPS-only, watchOS 26.2; iPhone 16, iOS 26.2.1; TV 4th Gen, tvOS 26.2 HomePods, audioOS 26.2

In general there is no localization for a country's app store, the language cannot be changed from the one set for that country.

This post https://discussions.apple.com/thread/255294216?answerId=259855941022&sortBy=oldest_first#259855941022 implies (but I do not see this confirmed) that in countries where there are multiple official languages (e.g., Switzerland), you might be able to effect a different App Store language if your device is set to that Region and then set the order of your preferred languages in Settings > General > Language, putting your preferred one at the top. Again, your Apple Account would have to be set to that country, you cannot just choose a different languages in Settings alone.