The Iconian empire ruled around 200 000 yrs ago. Their homeworld or maybe one of theirs was found in the romulan Neutral zone in 2365 by Captain Donal Varley of the starship Yamato. The Iconians were portrayed in ancient text as aggressors, and were said to be Demons or air and Darkness, but namy archaeologist and anthropologist speculated that the Iconians were innocent victims, attacked by enemies who feared their advanced technology. In 2365, a glimpse of the truth was explored briefly by Captain Picard of the starship USS Enterprise D. First, the planet had no lifeform, and also show multiple and heavy orbital bombardments that devastated the surface of the planet. The second find was a Gateway. Here is the best description given of this device. It's a sophisticated dimensional transport system, that allowed instantaneous movement across interstellar distances without the use of starships. The complex safe guarding the Gateway device, had still an active protection program of defense. This weapon works as follow. It's a destructive computer software weapon, delivered to an enemy spacecraft by means of a transmitter in a short-range space probe. The program acts like a virus and would start to alter the target vessel's computer software, causing failure of critical systems to finally destroy the enemy ship. Only one solution to eliminate the Iconian virus computer program, a complete shutdown of the ship systems, wipe out all memories and reboot from the ship backup memory system computer.
Only 2 Gateway were found, one in 2365 that was destroyed by Enterprise D to make sure it wouldn't into Romulan hands, and another in 2372 on the planet Vandros IV in the Gamma Quadrant. This one had his control mechanism and it's surrounding building destroyed by a joint mission with a Jem'Hadar force and a Federation force lead by Captain Sisko, there for preventing rebel Jem'Hadar soldier to take control of it.
To translate the language, it's required to have a program that cross the following languages : Dewar, Iccobar and Dinasian.
Addedum : Nothing more is know about them until recently when the USS Wolfheart commanded by Captain Ksour discovered and salvaged an Iconian ship. An anthropologists team and engineering teams had been assigned to learn about the ship system and database at a safe distance from Starbase Athena. A safe zone has been created, as no fly zone.