"te" is the second person singular indirect object pronoun. Normally this pronoun is "ti," but when the direct object of the verb is also a pronoun (here l') the indirect object pronoun is shifted to this variant form (ti -> te). I think the grammar books call it a "double object" pronoun.
mi->me ti->te gli/le->glie__ (glie actually combines with direct object pronoun)
ci->ce vi->ve gli->glie_
For a lot of excellent charts, explanations, and insight regarding pronouns and other Italian grammar topics, I recommend http://www.cyberitalian.com/en/html/gra_prpr.html
Also see Duolingo Italian Clitics-1 module
IMO (In My Opinion) it's more a question of education. If you have had a good grounding in English grammar and language skills, it shouldn't matter what region you're from. While there are certain minor issues over which grammarians can argue (e.g., subjunctive "were" is disappearing), if you know English well, you'll recognize how your regional dialect varies from Standard English. "That's the way we say it in [pick a place in the US]" is no excuse for bad English.
This is getting very confusing. I don't get this either. It seems that the translation should be °We had told you that.° or "We had told it to you." When the Italian has both the direct object and the indirect object, how can Duolingo leave out the direct object in the translation?
It's very similar to "It's my ball, so we're playing by my rules!"
At some point, we all leave the language nest and start doing our own translating, with the realization that we've done it correctly, even if there's another way of saying it in someone else's (i.e., DL's) opinion.
avevame detto = we had told
te = you. I suppose you are wondering about the > te l'avevamo detto = I don't know why they left the "it" out of the translation.
l' = it from some previous sentence that don't know about
l'aveamo detto = we had told it. the l' is the "it" from some previous sentence not know to us.
Sorry, but this is getting silly. the previous question, for which I lost a heart, contained "avevano detto". I put "had told" and the correct answer was "have told". This question has "avevamo detto" and I put "have told" only to be marked wrong and the correct answer given was "had told"! Can someone enlighte me please?
I have just been told to put "We had told you it." This is appalling English!! I had written "We had told you about it", which is at least clear. Once again, DL tells me that I do not know my native language, which is so utterly infuriating. I am trying to learn Italian, not be penalised for being unable to guess DL's twisted idea of English!
"We had told you it" is correct English and answers a different question to "we had told you about it". e.g.
"Why didn't you tell me about the wedding?" "We had told you about it but you weren't listening!"
"Why didn't you tell me [the date/the colour scheme/the address]?" "We had told you it but you forgot!"