If you can't handle advanced vocabulary and complex structures (grammar/syntax)in your mother tongue you won't be able to handle them in a foreign language either.

They will have to be mastered in both languages at the same time, which is difficult, time-consuming and quite disheartening.

Thank you a lot Christina for your point of view that I absolutely agree. If you have "X" knowlege in your native language, you cannot in any event exceed it when you learn onother language. Your level in your native language becomes your reference to learn the othors ones.   (Si je dis cela est évidemment sous un certain angle). Thank you again.