language_attributes() print WP lang instead content lang.

[shadowhck]

If you have WordPress in a language does not mean that the content has to be in that language. It would be good if there was an option to select it.

Another detail is for example: If we have the translation "Spanish - Spain", "es-ES", WordPress print "es-es" instead of "es" simply.
This would be good because it is the only translation into Spanish.

[nacin]

WordPress language values are designed to be only as specific as is necessary. Specificity is fine if there is a reason to distinguish among regions and subtags. es_ES is the code for Spanish spoken in Spain because Spanish is spoken in many other countries. WordPress, for example, also has Peruvian, Venezuelan, and Chilean translations, and I would not be surprised if Central America is represented in the future.

If you have WordPress in a language does not mean that the content has to be in that language. It would be good if there was an option to select it.

I agree, but we're not there yet. For now, this is best left to a plugin — for example, ​http://wordpress.org/extend/plugins/admin-in-english/.

[chriscct7]

As there are multiple Spanish translations now, this specificity actually solves a problem in which Spanish translation to use.