Your English is great, no problem
Not many people use nutritionists so this might not be 100% accurate, if anyone else has a better understanding then please correct me.
Another questions... The effect of the trainer specialty, and the nutritionist is cumulative?
Not sure on this, see:
(294450.105). It would make sense for it to be cumulative though.
The endurance of the players allways drops at the same rate? Or when it's higher it drops faster?
I believe all skills levels are equal distance, e.g dropping from 7 to 6 is the same as dropping from 2 to 1. I think you also see a very quick drop from stamina 10 to stamina 9, because stamina 10 = 10.0, there are no sub-levels. I've never had a 10 stamina player so can't confirm that though.
You lose a stamina level every 20 weeks or so (without any stamina training or nutritionist etc). Not sure if it's faster for 34 year old and older players (as they naturally decrease in all skills).
Which lvl of nutritionist do i have to contract to be able to increase the resistance of my youngest players?
I believe that, roughly speaking, level 6 will maintain stamina level and level 7 will increase stamina. Lower nutritionist levels will slow the rate of decline.
For the 18 ans 19 years old players, it's best to have a trainer lvl 4 and a young trainer lvl 4 or a normal trainer lvl 5? Someone knows exactly the difference?
I've previously read that each level of normal trainer gives +3%, and each level of youth trainer gives +1%. So the level 4 normal + level 4 youth would be slightly better.