Translating the website to more languages

[Outvi V]

Since suisei.moe starts to gain international reputation (thanks!), we plan to translate the site to more languages (such as fr) as well as more specific languages (such as zh-hans, zh-hant).

And what we need is:

• Fetch the language tag as specific as possible
• Provide rollbacks to more generalized language tags (like zh -> zh-hans)
• Add a language selection menu on page

In the meantime, we are accepting (and appreciating) contributions of more languages on sound translation or interface translation.