If translation for specific locale code such as en-US isn't found it
tries more generic en. When both are present, the en-US messages will be merged
over en. See [[loadFallbackMessages]] for details.
If the $language is less specific than [[sourceLanguage]], the method will try to
load the messages for [[sourceLanguage]]. For example: [[sourceLanguage]] is en-GB,
$language is en. The method will load the messages for en and merge them over en-GB.