Since this is a low magic campaign there maybe some nonmagical solutions to your problem. Most dwarves speak common (and maybe even elven); most dragons speak common or elven, wizards/sorcerers get draconic as a bonus language; im sure a high percent of the most common underdark creature (the drow) probably speaks either elven or common or both. I am sure you have thought of this so there may be some other reason why you want the PC's to know dwarven, undercommon, and draconic (other reasons aside from the ability to actually speak with these creatures, since presumably the PC's can use common to speak with dwarves, drow or dragons).