[Helborn]

For simplicity's sake I use "Common", which everyone knows at no cost, to communicate among a group. This is in addition to their own language.

I base it on the same principle as pilots today who have to know a smattering of English to communicate with towers. Common is a trade and mercenary argot. It allows basic communication but not reading or writing (even with Literacy). Literacy is useless for common because it has no written form. You're not going to discuss the philosophy of war in Common, but you might be able to discuss (in a simple manner) the relative merits of camels vs oxen or katana vs. broadsword. Realistically, "Common" should differ in different areas of Cidri, but this is a game, so it doesn't.

This eliminates the requirements as set forth in ITL RAW and in Tollenkar's Lair, but it simplifies dealing with early characters of different backgrounds. EVERYONE knows Common, in one form or another. It also means that a spy, or even experienced adventurer, is going to want to know a number of languages because characters from similar/identical backgrounds will speak their Native language to each other rather than Common.