[manifest] Add a language tag (#322)

Elsewhere, Addison Phillips, pointed out:

> Language: you still need to provide language information about the manifest's natural 
> language content items. Among other things, correct font selection rides on providing 
> language metadata to the text drawing layer. Inferring the language of the text by sniffing 
> the bits is inaccurate or impractical. The best practice is to provide this as additional 
> data.

I'm inclined to add a `lang` member. 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/322