There are two easy ways to handle this, depending on whether you want the readers to understand what is being said, or not. In truth, whether you want the readers to understand or not is the only important factor. Which characters understand is irrelevant to how you write the dialogue.
- If the reader isn't supposed to understand what is being said, you'd go:
Alice said something in French to Bob, that Charlie couldn't understand. Whatever it was, it appeared to make Bob happy.
- If the reader is supposed to understand, you go:
Alice turned to Bob. "It's going to rain," she said in French. To Charlie, who spoke only German, she said "I've just told Bob that it's going to be a sunny day."
The key element is: if the reader is supposed to understand what is being said, you write it in English. If it is said in a language other than English, and this is not the normal state of affairs within the story, you inform the reader that the particular line of dialogue is not being said in English. (If the whole story takes place in France and involves French farmers, it is obvious all dialogue is in French - you don't need to be telling this.) If a character cannot understand what is being said, you state that.
If the reader is not supposed to understand what is being said, then it's "something", a noise. You can describe that noise (what it sounds like), but since the reader isn't supposed to understand, there's no actual line of dialogue.