Complex sentence: The particular meaning that the term "nationalities" was to acquire in Spanish politics in reference to regions created some confusion with the concept of "nationality" in reference to citizenship, especially when the latter was defined in the 11th article of the constitution.
Split into simpler sentences: The particular meaning that the term "nationalities" was to acquire in Spanish politics, in reference to regions, created some confusion with the concept of "nationality" with regards to citizenship. The matter was especially confused when the latter was defined in the 11th article of the constitution.