You should know a lot about your character going into a roleplay, and if you don't, build on them as you're roleplaying. Give them enough personality for interesting development

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Have a plan for the development of your character. Give them a change.

Playing an asshole character? Give them a moment of kindness
Playing a shy/cowardly character? Plan a moment where they get a sudden burst of confidence or bravery.

Don't keep characters static. Make them change

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Like I said previously, base your character off yourself or someone you know.
Or, stick to what you're good at. I like to play comic relief characters and other people seem to like when I do as well. If you have a certain personality type(s) you're good at being, stick to that and tie all your character designs around that.

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Give your characters' backstory! It doesn't have to be deep or tragic. Just little things that would make them more interesting. Crazy things or habits they did as a child. A gnarly scar with a wild tale. An embarrassing teenage memory. Give them stories to tell.

Extra:
If you're going to have a character speak a language you're not familiar with, for the best, have the character speak mostly speak in your native language and if you plan to use a translator, use short, simple phrases and statements that can be easily translated. It keeps the immersion of your character, allowing them to represent themselves without botching the language of an entire group of people and possibly coming off as offensive.