Single commonly known words seem to be okay (there's a line of CW shoes called Senorita Shoebox). If you slipped up you'd probably be better off editing it out of your post, but at most I'd think you would get a warning.
I got a verbal warning by using the true name in french of a very very very famous painting, so super famous that I thought everyone would know. I was lucky and was able to appeal the verbal warning a year later to get it expunged. However we only get one account here on Subeta, so I advise caution.
I'd edit what I wrote, if I found that I had used another language accidentally with the idea in my head that everyone would know it because (apparently) our mods ...don't? Or aren't allowed.
Ok thank you and I sent two commonly known phrases to a friend in their profile comments a while ago. I'll be sure to correct any slip ups on the forums in the future. I was curious because I saw someone use a french expression in the shoutbox and remembered that they only allow English on here and that I had done that before, too. Thank you very much for the answers!
The very, very basic rule of thumb tends to be that if the word or phrase would get its own entry in an English-language dictionary, it's common enough to be allowed. However, the UAs have a lot of discretion in enforcing the rule.

Alrighty, thank you that's good to know!
The Dutch word "Can" in the sentence "You can..." is not a good word because it's too similar with an obscure English word.
It will give a lot annoying misunderstandings.
the most safe way to communicate is only with English. Even per smail it's better to use English with a member who knows your mother-language.
You can use other languages over smail, just not on public parts of the website :)

I dont even speak spanish but tend to out of silliness say 'very good/much' and such things irl, so it happens from time to time here. I always have to remind myself it isnt english even though I use it as if it were/because most people understand it.
Japanese words are especially hard to remember not to use with my old days of weebness/thinking everyone online understands. At least for now with my German learning I dont think in Japanese as much. Still it can be hard. But I err on the side of caution as I love my account too much and am paranoid xD
Still the hardest for me is probably just using common phrases from any language even if I dont fluently speak it. -hurdur-