>[code review feedback] is like getting a free-of-cost personal guidance about how to write good code
It's not free for the reviewer who's trying to safeguard project quality. If they're investing in you, it's likely under the assumption that you'll contribute back. If you don't, then you're just crowdsourcing and then running off with the ill-gained knowledge, possibly having had a negative impact on the project.
If you're unfamiliar with programming language idioms, it is likely that your (ultimate, approved) contribution may not be be worth the time and the effort that has been put into guiding you to it. The help may instead have been given to you in hopes that it will pay off with your future contributions. If your only or main motivation is to learn a programming language, there's a greater chance that there won't be any.
Contributing out of sheer sense of obligation is hard. Even worse, it may so happen that you realize that you don't even like the language. Not only is the the time invested in teaching you the project a waste for the project, but the teaching you the language was not beneficial for other projects written in the same language. To stay interested in a project, you should be certain of your interest in some thing intrinsic to the project.
At least be very frank and clear about your intentions.