Would a native speaker use "experience" in this context?
"If you want to learn a language, it's a good experience to watch movies in English."
It wouldn't be unreasonable to use "experience," but you would omit "a" so:
If you want to learn a language, it's good experience to watch movies in English.
Although, "practice" would be more common:
If you want to learn a language, it's good practice to watch movies in English.
Lastly, since watching movies in English only helps you learn English you wouldn't want to say the general term "a language" since the sentence only applies to English.
Best version:
If you want to learn English, it's good practice to watch movies in English.