It is an excellent question, and well constructed.

Thank you, Rachel.

Hi Iris, I agree with Phil, and would add another suggestion: 'How much time each day should a person commit to using a foreign language?' or How many minutes a day should a person commit to using a foreign language? I prefer to keep the time adverbial together - otherwise it could sound like you are referring to the use of a (different) foreign language a (each) day.

Thank you, Phil. Then it's the thing that only native speakers feel. I just made a question to the original sentence from BBC "We hope that by committing around 15 minutes a day to using it, a learner will see a real improvement in their English knowledge and skill."

It's correct, but for some reason I can't understand, I would use "per day" instead of "a day."