Language tagged strings and i18n

I noticed that language tagged strings are already supported internally in Cayley. It would be nice to be able to use this functionality through the HTTP APIs somehow (mostly Gizmo and GraphQL). I'm not entirely sure yet what would be the best way to include this. I would think it work somewhat similarly to the labeled sub-graphs, so I could imagine a `Lang` directive for Gizmo and a `@lang` and `@language` directive for GraphQL (similar to what I proposed in #614). Maybe it makes more sense though to have it work like the `@opt` directive where it doesn't propagate implicitly to the nested query. In any case, curious to hear what you think about it!