Fix translations for sites with an explicit lang

[mynetx]

When generating a multisite install, the generator sets the app locale to the site's short locale. Statamic's own Localize middleware uses the site's lang instead, which falls back to the short locale when not explicitly set.

So a site configured with an explicit lang — e.g. locale de_CH, lang de_CH, with translations in lang/de_CH.json — renders fine when served normally, but during a static build Laravel looks for de.json and the raw translation keys end up in the generated HTML.

This makes the generator match the middleware and use $site->lang(). Sites without an explicit lang behave exactly as before.

This also explains #166: when the short-locale lookup misses the site's translation files, Laravel serves the fallback locale's strings instead — so pages come out in the wrong language (or a mix, when only some keys fall back) rather than as raw keys. Reproduced on 4.x with a fr_FR site and PHP lang groups: the live site renders the French strings, the static build renders the English fallback. With this fix the static build matches the live site.

Fixes #202
Fixes #166