Enable Language and Locale module, add a language, and enable URL detection

[olafgrabienski]

As a first step, to make Backdrop4Good a multilingual site, we want to

• enable the Language module,
• enable the Locale module,
• add German as an example language,
• configure language negotiation.

These steps are needed to translate the user interface.

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PR: https://github.com/Backdrop4Good/backdrop4good.org/pull/20