Add a simplified Chinese translation

[qyearsley]

This PR includes:

• Move the existing Chinese translation to values-zh-TW (The Taiwan Chinese locale, often used for traditional Chinese).
• Add a simplified Chinese translation in values-zh-CN (The China Chinese locale, often used for simplified Chinese).
• A minor improvement to the terms "unit", "imperial", and "metric" in the existing translation.

The simplified Chinese translation was obtained by running opencc to convert the original translation to the simplified one, and then manually replacing all of the terminology that differs between mainland and Taiwan, including:

• app 应用程序
• bok choy 小白菜
• avocado 牛油果
• sweet potato 番薯
• kiwifruit 猕猴桃
• lime 青柠

[qyearsley]

@slavick Is creating a PR the recommended way to contribute, or is there another different way to contribute to this project?

[slavick]

Yes, a PR is the recommended way to contribute. Translations are not a priority at the moment as many of the strings will be changing.

[qyearsley]

Got it, after the strings have been changed and are more stabilized, I'll re-send this.