protected function prepareCacheId($locale, $domain) {
		if (is_array($locale)) {
			$list = [];
			
			// many locales to many domains
			if (is_array($domain)) {
				foreach ($locale as $part1) {
					foreach ($domain as $part2) {
						$list[] = $part1 . '_' . $part2;
					}
				}
				
				return $list;
			}
			
			// many locales to one domain
			foreach ($locale as $part) {
				$list[] = $part1 . '_' . $domain;
			}
			
			return $list;
		}
		
		// one locale to many domains
		if (is_array($domain)) {
			$list = [];
			
			foreach ($domain as $part) {
				$list[] = $locale . '_' . $part;
			}
			
			return $list;
		}
		
		// one locale to one domain
		return $locale . '_' . $domain;
	}