Are "mersi" and "mulțumesc" equally polite ? Because once I wrote "thanks" for the translation of "mulțumesc" and it was wrong.
Hit the report button next time you see that. This particular course is made/maintained by volunteers, so they didn't put every possible correct translation into the database.
So some right translations aren't marked correct, and the way you get that fixed is you report the problem.
As I wrote above, this is a volunteer-run course. The volunteers have to type in every accepted translation. That means they don't have every correct translation listed for every sentence in the course. Next time you do this lesson type "Thanks! You're Welcome!" and if it's not accepted hit the Report button.