i18n/zh-cn: Add Simplified Chinese translation

[dreammu]

Some words are difficult to find appropriate translations for in Chinese, but the ones provided should be enough.

[bastimeyer]

Thank you very much for the effort of adding a new translation, @dreammu. Much appreciated.

There are two minor issues in the locales.json file which I've annotated. I also have two questions regarding the translations themselves.

I obviously can't comment on the translations, but if there's anything which was unclear during the translation due to missing context, etc., I'll be happy to help clarify.

I also checked for any layout issues, but everything looked fine. The language selection in the settings menu is currently not ordered correctly, but that's because of 17f01c2 which I will fix after this one got merged.

[bastimeyer]

One pluralization issue left. Once fixed, I'll have a final review later today and will then merge.
Thanks again.

[bastimeyer]

Thanks again, @dreammu.

I locally rebased your commits onto master for final testing, squashed the commits and then force-pushed onto your branch, so that I didn't have to squash via the GH interface.