More casually, you can also say (with X, your unknown word) Qu'est-ce que X veut dire ? Qu'est-ce que ça veut dire, X ?

You say: Que signifie le mot (insert word) ? And it should be "en français" without any article. The article L' stands for LE or LA in front of a word beginning orally with a vowel. Here it's not possible to put L' before français. Name of languages are masculine so LE français (but not needed here).