It is not a good idea to mix two languages within a single page. Google uses the words that you use to determine which language you are writing in. Then it uses your site in the search engine result pages (SERPs) only for users who speak that language. When you mix languages, it confuses Google in this regard. Google asks that you not mix languages in a single page.
Avoid mixing languages on each page, as this may confuse Googlebot as well as your users. Keep navigation and content in the same language on each page.
In the case of a word here and a word there that your users are likely to understand, it shouldn't confuse Googlebot too much. Here is a report from a blogger who is unhappy with that policy from Google and ranks fine using an occasional multilingual word or phrase:
I’m really disappointed to see this kind of advice handed out. Yes, it confuses Googlebot, but only because it doesn't (I guess?) take into account lang=”xx” attributes. (Yeah, nobody uses them, but that’s because nobody parses them.)
But users? Most people are not pure monolinguals. We need ways to make linguistic barriers online weaker, and not stronger.
I've been mixing languages on Climb to the Stars for eight years now, and it hasn't prevented my readers or my Page Rank from being happy.
I see that you also asked this question in the Google Help Forum where Google's John Mueller says:
Doing that is fine and not something to worry about.