Generic strings (website-base in Weblate) not shown in Simplified Chinese due to mismatched language code

On https://gajim.org/zh-cn/, most of the strings are displayed in English instead of Simplified Chinese. After renaming i18n/zh_Hans.json to i18n/zh-cn.json and building Gajim’s website locally, the issue was resolved. It would be better to update the corresponding translation file both in Weblate and repository to avoid this problem.