How do you think will it look and what languages will they speak
1) 'Spain' didn't merge during the Reconquista (except at the very end, when it was essentially over), and even then not completely, as Portugal is not included today ('Spain' pre-Ferdinand & Isabella referred to Iberia, not a single country)
2) France (i.e. the King in Paris) took a LONG time to exercise control over all of what's now France
3) 'French' is the language of Paris (with only trivial oversimplification). IIRC, 'Spanish' is the dialect of Madrid.
How does this 'merge' happen? I can't imagine a King in Paris having the concentration to push the Reconquista. OTOH, a King in Madrid or Oporto/Lisbon would probably not be able to establish rule over 'France'.
If the King doing the uniting were based in Toulouse, say, or Navarre or Barcelona, then you MIGHT be able to pull the thing off. If so, then Normany probably belongs to England/Britain, Brittany is independent and Burgandy is a large country, and Savoy extends further into OTL France.
If the King is in Barcelona or Toulouse, then the court language is some variant of Catalan/Occitan (which some consider varying dialects of the same basic language).