I'm not sure I understand why you would expect to be able to do this with a language you are not very familiar with, although I also am not sure exactly what not very familiar means here.

I hope however if you are going to analyse a codebase in a language you are not familiar with one of two things apply -

1. There is a part of the codebase that uses some language you are very familiar with, or that touches a specific knowledge domain (like frontend development or database querying) that you are very familiar with, if so start at the point of greatest familiarity and try to walk backwards and document for yourself bits of code that are touching the stuff you do know well.

2. You are in a situation where you are a secondary developer to someone who is an expert in the language, hopefully they can give you a tour of the codebase.

If neither of these are the case I guess you are going to have to learn the language and get a beginners guide to Flask or something like that. Maybe ask on a forum for the language what the best learning sources are.