I was reading a review on Amazon which said the following:
To sum up, this method works fine for me and I can recommend it wholeheartedly. But then again, there are always people who love to sweat...
What does sweat mean here?
My best guess is that sweat refers to doing hard work. When you do hard work physically, you sweat, and so when you do hard work mentally, you figuratively sweat. This is sense three in Macmillan:
[intransitive] informal to work hard
Why do I interpret it this way? Well, the review starts with this line:
The Assimil method is a great way to build up your language skills in an easy and relaxed manner.
Those people who "like to sweat" are people who like to do things the hard way. The reviewer is characterizing the Assimil method as easy and relaxed, requiring less work.