Prompted by the comments to this question
Bearing in mind the FAQ on this subject, in particular:
Unless you're posting on a language-related site (e.g. French Language) or a site where all questions are expected to be in a different language (e.g. Stack Overflow in Spanish), yes, all posts are expected to be in English.
If someone makes a non-English post on a site other than one in the above two lists, or in a language different from the site's accepted languages, first, check to see if it's spam. A small portion of wrong-language posts are actually spam, so be sure to check for that.
If it's not spam, vote or flag to close it as "Needs detail or clarity" if it's a question, or flag as "very low quality" if it's an answer.
...it is not necessary to translate wrong-language posts. Machine translations (e.g. Google Translate) can be inaccurate, and even human translations risk distorting the intended meaning of the post. It's up to the author to make sure that their post fits the quality standards of the site; if they don't, it reduces their chance of getting a good answer (in the case of a question) or that their post will be well-received.
What is the policy, or preferred convention, when a post includes non-English content? i.e. one that is only partially non-English so falls outside of the above "vote to close" criterion?