I don't know what more to suggest. Something is breaking the translation of CB Quickstart language strings. Please PM backend super user login credentials and will take a look.

Do the language overrides apply to the custom fields I have created?

Language overrides apply to field Title and Description. For Select fields it applies to the Label as well. Stored user values do not translate (e.g. a text field value is not ran through translation API).

They are not been translated, like.. field names and balloons help pop-up from those custom fields. Where can I enter the translation for each CB language for the customs fields I have created?

Those are translated using language overrides. Everything except core language strings is translated through language overrides.

Those strings are all translating fine to the default strings. I've tested every language you have available in your language switcher at the top of the page. If you need those strings translated in other languages you will have to do that yourself using language overrides or contribute to the language projects on Transifex to have them translated. I'm not seeing any issues with raw language keys outputting like in your original post. If you're still having such an issue you'll need to provide steps to reproduce the problem.

Before I invest a great amount of time with it I need to understand the language override. May be it is simple for the creators, I just need more information to apply it and the link does not give me a clear application for what I am doing. Just want to make sure.

Edit a language plugin in CB > Plugin Management then click the Overrides tab. A language finder is provided for searching for language keys. Example override as follows.

Key: PROFILE_ACTIVITY_FILTER_GROUPS
Text: Groups