Inconsistencies of wording for the default language / Locale

The inconsistencies are:

1. In the Information Tab -> Locale
2. In the Page Information Panel (wiki editor and standalone WYSIWYG editor) -> Default Language
3. In the Localization section of the Admin UI -> Language terminology (vs Locale) -> "Language", "Supported Languages" and "Default Language" (and "Multilingual")

We need to:

1. Decide if we want to use the Language terminology or the Locale one (which seems more technical for end users IMO)
2. Fix the inconsistencies to use either Locale or Language and don't use the "Default" prefix in some editors and no prefix in others