When it comes to the use of foreign languages on MLPF, the key word to remember would be 'inclusion'. The bottom line is to keep it to a minimum when at all possible. After all, this is an English speaking site with English speaking staff. The more foreign language used, the more other members are liable to feel excluded from reading or even participating with that content, and at the same time, the harder it is for staff to do their jobs. How can one moderate something they can't read?
To break it down for you:
- Citing common phrases, sayings, greetings, etc. is always okay. Greetings, short sayings, phrases and other brief statements is fine, regardless the language.
- Uncommon phrases (e.g. just writing out something from another language) are (usually) okay. They should contain a translation if at all possible, but are generally fine.
- Speaking in complete sentences in another language is not okay, anywhere. Even if you're kind enough to include a translation, the staff would really prefer if people would refrain from doing it in the first place.
Extenuating circumstances, such as academic purposes, negate the above guideline. For example, it would be rather silly to forbid discussions centered around translations of Dante's Inferno, would it not?
These rules of thumb for other languages are held in a global respect. That means posts, signatures, statuses, blogs, PM's, and every other nook and cranny on the site where one can type something and have it viewed by members, falls under these guidelines.