"¿Tú cuándo comenzaste a estudiar francés?"
Translation:When did you start studying French?
Yes, it is proper English in Britain, North America and anywhere in the British Commonwealth.
But Spanish uses the infinitive (estudiar or "to study") to express what is often conveyed in English with a gerund ("ing" participle used as a noun or "studying"). So DL probably wants "start studying" here so we learn how that concept is expressed in Spanish (with an infinitive, estudiar).
Your version is a nice word-for-word translation, which works perfectly well with this sentence, but won't work with others. DL is trying to make sure we know to use infinitives instead of gerunds in Spanish.
IMHO one can "study" a language, i.e analyze it, its workings, etc. and one can "learn" a language in the hope of acquiring sufficient knowledge to hold a conversation. In this instance I would hope the concerned individual (just like us) is trying to acquire enough knowledge of the language to someday speak it fluently. "When did you start learning French" should be accepted.
Sorry, but no. I don't disagree with the way you want to define the terms, but that's not the point. "When did you start learning French" requires a different verb.
When you play darts, I guess you think you should get points for hitting the waitress serving drinks near the target.