Why is linguae Latinae used here instead of the linguam Latinam because Marcus is male (presumably)?
Nothing to do with Marcus.
It is because studeō, studēre (here studet) takes the dative (linquae Latīnae) as the 'direct object', instead of the accusative (linquam Latīnam) like most other verbs do.
I hope the developers can put that information into the grammar section for the level.
Thank you. That's very helpful. :-)