Sort out the xx vs xx_YY language mess

Back in June 2005 we decided to introduce a firm xx_YY locale notation and always use the country (even in cases where it's not needed). It was a good decision (it makes a lot of places simpler), but we need a xx → xx_YY mapping, so that pl maps to pl_PL, hi maps to hi_IN, es maps to es_ES and en maps to en_US.

We should then use this mapping wherever needed (say, when we inherit Drupal's language or set the preferred user's language), and stick to using xx_YY notation internally (say, in civicrm_contact.preferred_language).

We should integrate CRM_Core_I18n_PseudoConstant::languages() with the relevant civicrm_option_group.