Add extra check to handle formal and informal locales.

The wp_locale in the translation_sets object do not contain the _formal or _informal part. For example: the wp_locale of Dutch (Formal) is nl_NL. The information that it's formal is only in the `slug` property. Because this->locale does have the _formal or _informal part, the comparison with the wp_locale had an incorrect result for informal and formal languages. This commit adds an extra check in which the slug is appended to the locale if the slug is not 'default' (i.e. is 'informal' or 'formal'). This string is compared to this->locale to make sure the correct $set is returned.

Author: IreneStr

src/i18n-v3.php

@@ -388,6 +388,9 @@ private function retrieve_translation_details() {
 					return $set;
 				}
 			}
+			if ( $set->slug !== 'default' && strtolower( $this->locale ) === strtolower( $set->wp_locale . '_' . $set->slug ) ) {
+				return $set;
+			}
 		}
 
 		return null;