What I'm understanding, is that the 12th word can't be predicted from the 11 first words. I need to choose a random 12th words, cumpute something, getting the checksum and chose the words that validate my seed ?
Yes, If you have 11 words, for finding a valid 12th word, you have to brute-force it. Because for computing the checksum, you need the first 7 bits of the 12th word as well.
It's not a good idea to select 11 words and find a valid 12th words. The best way is to select the first 128 bits (preferably using a dice), calculate the checksum and then find corresponding words.