fix(citation): Language parameter for non en-US settings (#2075)

* Fix language parameter of the citation plugin for non en-US settings

Per default the rehype-citation project only supports en-US, as
explained here: https://github.com/timlrx/rehype-citation/issues/12
For other languages one can provide a locale-file either by passing
its path or providing an URL. The following repository contains locale
files for multiple languages. So, these are used, in case a non en-US
language is used in quarzt. But this optimistically assumes there is
indeed an according locale file.

In summary this solves the problem only partially, since there are
still some languages which will not work properly.

* Fixing code style by running prettier with --write

* Excluding `en-US` locales from the new behaviour.

* Removing unnecessary `null` und `undefined` check.

* Update quartz/plugins/transformers/citations.ts

* Update quartz/plugins/transformers/citations.ts

* Update quartz/plugins/transformers/citations.ts

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Co-authored-by: Jacky Zhao <j.zhao2k19@gmail.com>