In many, many English-language novels I've come across characters who speak other languages, most often French for whatever reason.
In many cases, such as with Ulysses and Lolita, this is a simple matter, because the author was multilingual. He simply wrote the characters himself.
But that can't always be the case, because even someone who speaks French, English, Spanish, Russian, and Hindi might want a character who constantly references his Japanese heritage.
There must be time when authors want to include characters who speak languages that the author does not.
How is this dealt with? Do they perhaps write the content of what is to be said, and then find another professional to help them?