Changing translation handling from gettext to babel
Changing translation handling from gettext to babel

Despite the fact that Flask-Babel is already configured, the translation
was done with the builtin gettext-library which had no clue about the
preferred language in a request.
By switching the `_()`-alias to the `gettext`-function from Flask-Babel,
the preferred language will be used in each request.

This fixes #1065

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