When this happens, it's usually not practical or possible to use a dictionary. For this reason it is important to explicitly teach the skill of using circumlocution. Circumlocution is a strategy for describing or defining a concept instead of saying or writing the specific words. Students should be taught how to so this and the teacher can give them tools to help in the process.
This document has a clear and concise explanation of circumlocution for for students as well as many examples of phrases to use to convey meaning when the exact word is not known. This is followed by two partner activities that students can use to practice the strategies. The document is written in accessible French.