Spanish could be ESP or SPA (find out which one is a better fit for your users).
Basque could be BAQ or EUS (make sure you select what the Basque peoplepopulation would recognise).
Important: I would use EUS short for Euskera only if the Basque peopleBasques recognise Euskera as Basque. Remember that you are providing the language option for the Basque population not the Spanish speaking one.
References:
https://en.wikipedia.org/wiki/Basque_language
https://www.loc.gov/standards/iso639-2/php/langcodes_name.php?code_ID=47