Google’s John Mueller said that Google Search tries “to find the primary language per page.” So when you mix languages, that might cause issues with Google doing that. He added on Twitter “but what you put on different pages is totally up to you.”
Here is that tweet:
Sure, no problem. We try to find the primary language per page, but what you put on different pages is totally up to you.
— 🍌 John 🍌 (@JohnMu) July 11, 2021
Now, John said this numerous times before. In 2010, yes, over ten years ago, John Mueller talked about Google’s one language per page rule. In 2014 he described examples of where mixing languages can cause issues in Google Search. In 2018, John said try not to mix languages in your title tags.
I am sure I’ve covered this other times but it seems like it is best to try to stick to one language per page, not just for SEO purposes but also probably for your website visitors.
Forum discussion at Twitter.