What does the checkbox in the Languages List do?

Hi,
The checkbox at the end of each language is to Enable / Disable it.
You can use disabled languages to store data for you game or more commonly, to avoid showing some languages in your builds, if you are still developing them.

About the default language. What do you mean by that?

Here is a doc with details about the default language: (the one that will be selected automatically unless the user changes it manually)
inter-illusion.com/assets/I2Localization...InitialLanguage.html

If by default language, you are referring to the one that its used when you click "translate" or "translate all", then that depends on the order you set your languages. Normally, when you click translate, the plugin will try finding the first language in your term that its not empty. And it will translate from that language to the one you clicked. If no language is found, then it will translate from the Term's name into that language.

Hope that helps,
Frank