Khazalid

Khazalid is the native language of dwarves. Both in the West and the Old World, most dwarves learn one or more human languages, but they still teach their own tongue to their children first and foremost.
Khazalid is not exactly a secret language per se but very few non-dwarves bother to learn it and dwarves are not especially eager to teach other races their languages without sufficient incentive.
Khazalid is vaguely similar to real world Old Norse.