Add document for lang setting

[hashrock]

It can be changed by setting ctx.lang = "en" in main.ts, but it is better to mention it in the documentation.

• Create FAQ section
• or, explicitly specify lang="en" in a generated main.ts

[vjdv]

I'm getting ctx is not defined, do you have an example?

[hashrock]

@vjdv
here you are:

https://github.com/deno-ja/deno-ja/blob/762dcb5a9cf0bbfcda8bcfaa59b1f65d6a70674e/main.ts#L16