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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.
Here are the tweets:
Text in alt attributes can be seen as a part of a page - mixing language makes it harder to determine which language is the primary language of a page. Ideally you'd translate it all :)
— John ☆.o(≧▽≦)o.☆ (@JohnMu) January 24, 2018
I can't make that decision for you - a part of the page is translated, a part isn't - you need to decide how you want your pages to be seen.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) January 24, 2018
There has been some conflicting advice from John on this in the past. In 2010, John said make sure the language on the page is not mixed, that you use one language on a single page. Then in 2013 he said if you do mix languages on the page it is fine and he brought it up again in 2014.
Your best bet - try to give Google clear signals about your page and that includes the language.
Forum discussion at Twitter.
