Real world translations
I'm not sure if I get how this thing works. How do I know if my translations are 100% correct and how come those translations are needed to master a level if the difficulty of those texts is beyond the level itself?
Translations can hardly be 100% correct. They can be wrong, but those which are right can be both good and mediocre. You can look at the "current best translation" or at the other translations people made and compare them with yours.
"Mastering" levels is not mandatory. You can go on down the skill tree and return to translations when you feel confident enough. The whole point of this site is to teach you to read in a foreign language and make use of your translations. Sooner or later you will be quite capable of translating :)