Translation:Is this your key? I found it yesterday.
"Is it your key ? I've found it yesterday" is not accepted, and the correction says "Is it your key, I'd found it yesterday", which doesn't feel ok to me ("had found" is a little too much in the past for "yesterday", isn't it ?), what do you native english speakers think about it, should I report the sentence ?
No, it shouldn't be accepted. The present perfect ("I have found it") isn't used with specific references to past times like 'yesterday'.
The main translation here uses simple past tense, and that's completely normal. The past perfect version ("I'd found it") is also accepted, and while that's not an obvious interpretation of the sentence, it's not necessarily wrong either. Past perfect isn't used for things which are far in the past. It's used when you want to say that something happened before something else. In this case there is no context, and we can only guess what the 'something else' could be. It could be our last conversation: I had already found the key before our last conversation, but I didn’t tell you about it at that time, because I didn’t know that you were looking for a key.