The English language was hardcoded to load empty translations for
templates as part of sphinx-doc#10067,
but the rest of the code is already capable of handling missing
translations just fine, so we can simply drop the first if block.

This allows to use translations for English in custom templates, as for
all the other languages.

Fixes sphinx-doc#12214

Indeed, English is the source language of the Sphinx project, so there
is no catalogs for it. Wee still need to keep the catalog loading for
English, in case some translated strings are defined in custom templates
in project which source language is not English.

…anslations-templates