I think those 24 languages reflect all the languages that are official languages at country level.
So for instance, Basque is not an official language of any country (only French in France and Spanish/Castilian in Spain). Belgium's official languages are French, Dutch, and German, "Flemish" is only a local variant of Dutch (Belgian French is also only a local variant of French).
Official is a weird concept though. Turns out Dutch law never really bothered to define an official language, Dutch simply is the de facto standard and is required for a lot of things making it effectively the standard. This makes Dutch Sign Language the only language officially recognised by law. An attempt to recognise Frysian and Dutch as official languages in the constitution failed.
Sweden didn't have an "official" language before the Language Law of 2009. Five minority languages (Finnish, Meänkieli, Romani, Sámi, Yiddish) were officially recognized as such since 1999.
In the US, people will resort to fisticuffs, over variants of Spanish. I usually translate into Castilian Spanish, because that seems to be the equivalent of "Vanilla" Spanish. No one is really happy (except the Spaniards), but I'm not accused of favoritism.
For what it’s worth, Castilian sounds very odd to American ears. For a good time you can ask «¿en castellano?» and be met with either a blank stare or laughter.
Basque is an official language and declared as such in the Spanish constitution however restricted only to the regions that decide to apply it (Basque Country and Navarra).
If we want to go all legal, I believe that Spanish/Castilian is the only official language of the State, so at country level, with the other "Spanish languages" only official in their respective areas:
Section 3
(1) Castilian is the official Spanish language of the State. All Spaniards have the duty to know it and the right to use it.
(2) The other Spanish languages shall also be official in the respective Autonomous Communities in accordance with their Statutes.
(3) The richness of the different linguistic modalities of Spain is a cultural heritage which shall be specially respected and protected. [1]
So for instance, Basque is not an official language of any country (only French in France and Spanish/Castilian in Spain). Belgium's official languages are French, Dutch, and German, "Flemish" is only a local variant of Dutch (Belgian French is also only a local variant of French).