Hello! Quick question. Can I use another language as part of a role play? It is important in the conversation as a character asks mine to speak a bit in another language?

It's perfectly fine as long as you include an English translation afterwords under the hidden text/revelio thing

If I reply as I explain to my friend what it means?

Okay so if you're talking about real life foreign language (french, spanish, etc), you can do something like this
"Comment vas-tu mon amie" How are you my friend or you could use a ^superscript and put a 1 by the dialogue then at the bottom of the post just translating it.

If it's a fake language like Part Goblin's gobbledygook, you need to either be of the race or have comprehend languages and specify that language.