Can I add a language to the Google spreadsheet?

Yes, if you add a new column to your spreadsheet, I2 Localization will download it and add it as a new language.

The only other thing you have to setup in your project is that instead of showing fixed buttons to change languages, you have to read all the languages name from the source and show them in a popup or list so that the players could change into the new language.
If your project is setup that way, then adding new columns will make the new languages available to your players without having to do a new build.

Hope that helps,
Frank