"Then you would want to edit the RaceMasks (column 2) of SkillRaceClassInfo.dbc"- Incorrect, to seme extend. There are records which are actually preventing race/class combinations from ever being able to use certain skills.
Definitely not thoroughly explained enough, only possibly incorrect, it depends on what you know about that DBC . My experience is that making just a very short and even potentially confusing note usually produces just more questions, which could be answered in my first answer already, had I taken some more time to get more into detail. Sadly, you usually don't actually save time by doing this .
"AllowTwoSide.Interaction.Chat = 1AllowTwoSide.Interaction.Channel = 1"- Will enable Horde and Alliance members to communicate with each other, it will NOT teach them languages OR enable them to use those languages. Thats quite a difference.