Google: Translate Your Image Alt Tags As Well

Google's John Mueller said that having different languages on the same page "makes it harder to determine which language is the primary language of a page." You don't want to make it harder for Google to understand your pages, ever. So make sure to translate your page fully when doing that translation.
That includes the image alternative text attributes, alt tags. John Mueller said if you don't translate those as well, it can make it harder for Google to understand them as well.