I really like to use project euler when I want to learn a new language. Exercises are sorted more or less by difficulty, at least for the first 50-100, and are completely langage agnostic (you just need to answer a question in a text input).

It's also very good to learn algorithm and maths. Some problems can even be solved without any programming. And for some, a certain amount of math is necessary to have a efficient solution to the problem.

Ps: you can find the first 50 problems in french here : https://www.lucaswillems.com/fr/articles/19/project-euler-tr...