Translation:Let me introduce to you, this is my best friend.
Your sentence, "let me introduce to you, this is my best friend." is incorrect. It would be accurate to say, " Let me introduce to you my best friend."
Yes, this is incorrect English. "Let me introduce to you" is bad form. You need to drop the "to" in that phrase.
I'm not native to English and i feel that the more i play this game the worst my English has become. It's like a trade-off
"I will introduce you. This is my best friend." should be accepted.
The first clause ("我給你介紹") is literally "I give you introduction." and would end up being something like "I will give you an introduction." in the future tense in practice in English.
I assume Duolingo uses this translation to keep the statement in the present tense, but they had to change the meaning and use "let" to do that, so allowing answers that use the future tense shouldn't be a problem.
The English is incorrect. It should be two sentences in this case. 'Let me introduce you. This is my best friend.'
我給你介紹 in Duolingo's defence, is addressing the person to be introduced. People are getting hung up on the tense. Think of this as explaining to your best friend how you introduce them in any given situation, not just in one situation (in the present or future). "This is how I introduce you: this is my best friend."