Bug 1078297: Need localized number formatting helper for jinja templates

Also change l10n_format_* helpers to use lang from context.
To me the way it is reads a little better and is more obvious what's going on. I'd guess it's also (very slightly) more efficient to do a membership test than to create a list and reassign the variable, though since `en-US` is the default, it'll likely go down that path a lot more than not. I'm cool either way.