getPreferredLanguage crashes with index out of range exception when currentLocale does not contain an '_'

the getPreferredLanguage function assumes that the returned value of [NSLocale localeIdentifier] contains an '_'. This may be true most of the time, but we have ran into issues where the localeIdentifier does not contain any underscores (could be due to an invalid region being returned?). In any case, we need to handle gracefully when there is no underscore present to avoid an index out of bounds exception