Giant is a unique language of some of the giants races. It may be a dialect of the Titan language, as mountain giants speak Titan, but may have also been spoken by some of the ogre tribes (ogres are one of species of Draenor giants). The ogre Mug'thol and the former human Marijah also know how to speak this language.
"Giant" has an apparent connection to DnD derived generic languages, which apparently can be found in DnD rule books. It is the language of ogres and giants.