Extend request-language handling

POM Translate exposes final request decisions rather than a filter that accepts any arbitrary resolved locale:

• pom_translate_should_handle_ajax_request( bool $allowed, array $allowlist );
• pom_translate_should_handle_rest_request( bool $allowed, string $route, string $method, string $context );
• pom_translate_should_handle_frontend_routes( bool $enabled ).

Use the REST and AJAX allowlist filters first. Reserve the final decision filters for integrations whose request shape cannot be described by the standard lists.

pom_translate_language_changed( string $new_locale, string $old_locale ) is an action fired after an explicit context change. Use it to invalidate request-local integration caches, not to redirect visitors or persist preferences.