which language

I would start with Spanish as Catalan is a regional language and might not be useful if you visited Madrid (for example).

I agree with Cooley.

If one was going to live in say Wales or Ireland,  whilst speaking Welsh or Gaelic might be OK, if only sneaking one language,  English  would be the only practical one.


https://en.wikipedia.org/wiki/Languages_of_Spain

]n terms of the number of speakers and dominance, the most prominent of the languages of Spain is Spanish (Castilian), spoken by about 99% of Spaniards as a first or second language.  Catalan (or Valencian) is spoken by 17%, Galician by 7%, and Basque by 2% of the population.

Both!
Spanish first. Then Catalan.
But do learn both.
Having said that, it also depends on what job you are going to try to get.
That's a whole other topic altogether.

Black.
             Having lived in Spain 30 years and been a volunteer translator with the police for 20 years,  I am surprised by your pòst,   

Hardly any Brits ever get passed ordering a coffee or a beer and they do that very badly even after years of having lived here.

But every best wish to Tug.