Which is the official site to download the NXT (nxtcoin) wallet app? Where is the official site to download the NXT wallet app?
There is no the official site.

You can download NRS (Nxt Reference Software) by following links from https://nxtforum.org/nrs-releases/. If you trust Jean-Luc then just check the file checksum. If you don't then analyze the source code and compile it yourself.