Polishing Simplified Chinese Translations

Hopefully this can (partially) solve #33 The original Chinese (Simplified) translation may be generated by machine translation and therefore have quite a few unnatural words and phrases. This PR synced the new keys from the English translations and further adjusted the translation. However, in the long run, it may be more beneficial to set up a translation platform such as [Weblate](https://hosted.weblate.org) to handle key-value synchronisation automatically. Manually comparing Json key-value pairs is not very efficient as the strings continue to grow.