You can edit the languages in the nation editor, I'm sure you saw that already. Please do not enter a value "0" with OOTP versions before 12.1.12, it has a bug that basically destroyed the league when you entered "0" for the language
The upcoming patch will fix this.
The numbers for the languages are only ratios, not percentages. For any nation, you can enter any numbers and the total does not matter at all.
If a nation has two languages, for example spanish and english, and there are 25% speaking english, than you could enter 25 and 75, or 250 and 750, or 4 and 12 or whatever.
The USA has to have the ID #206 and the name must be "The United States" at the moment. OOTP needs to find the USA record and that's what OOTP uses to find it.
Any other questions, please ask. I subscribed to this thread.