It is quite common to use more than one language for writing and reading. Very common. However, the reader feature of Edge is not detecting the language and it is quite annoying to hear the reader reading in a different language you want. The only way now is by changing the language of the reader to use the right natural voice for the language of the page.
I know it is quite challenging having a single voice able to read and pronounce correctly different languages, but detecting the language of the page is quite easy to implement. Depending on that detection Edge could use the right voice. At least until the reader is able to read pages using multiple languages.
Not related to Edge, but the same issue happens when trying to use your navigation software in another country.