Locale handling

[nylen]

We don't have locale-specific builds of ClassicPress yet, so all migration will need to happen using the default en_US build until those are ready.

I tested the migration process using an es_ES install (code added to support this in 68e57ca). It worked, but after the migration, the site's locale was set back to English.

I changed it to Spanish, and most strings were still translated. As expected, though, ClassicPress-specific strings were left in English:

Potential action items here, in order from easiest to hardest:

• Add a warning indicating that the site's locale will be switched back to English after the migration.
• Figure out why the site's locale is switched back to English, and prevent this from happening.
• Release localized builds of ClassicPress and use them in the plugin.

[nylen]

See ClassicPress/ClassicPress-v1#154:

It's probably best to address the overarching issue with this: #18 on the migrate plugin. Maybe a yellow caution icon and warning letting them know their current locale isn't supported yet?

When we migrate a WP site from a language that ClassicPress doesn't support yet (i.e. not listed in our translations API endpoint) then we should show a warning message.

Also need to test & make sure that WP language packs get correctly updated/refreshed to the latest ClassicPress languages during the migration.