I agree with you: contributing to Open Source is a great way to give back but also a wonderful way to learn many things (and not only technical).
Localising and documentation translation, especially for non-mainstream language, is always a very useful way to contribute. It is also a good way to learn about the product.
Where to start? It is a very good question
First advice I would give is to choose a topic that you are passionate about or you use everyday (a particular plugin for example). Pick a beginner issue in that field and start exploring it. Remember, the first step is to reproduce the defect. It will help you getting familiar with the product and understand the problem.
Another good way to start learning about Jenkins programing is to help modernize a plugin. A very good guide for that is described in Improve a plugin tutorial blog post.