i18n: use TG2 compatible settings names i18n.lang and i18n.enabled

Following changes are made:
- Remove list of available translations: severely outdated and hard to
maintain.
- Replace lang with i18n.lang, reasons:
- Follow TurboGears2 conventions
- Align with 'i18n.enabled' which cannot be shortened
- Scoped settings provide grouping and are more clear
The old 'lang' is still recognized though, so no upgrade issue.
- Introduce/document i18n.enabled

Comments are intentionally kept concise in favor of the documentation.