I guess the big question is whether or not Afonso V of Portugal would be able to achieve what Ferdinand of Aragon could not, and become full king of Castille with the right to pass that crown down to his descendents. Otherwise, he may still have an heir with La Beltraneja but that heir will only receive the Crown of Castille while Portugal would naturally go to Afonso's eldest son João II. Now, the reason why Ferdinand wasn't able to get his way IOTL was because the powerful Castillan aristocracy (which had supported Isabella's claim to the throne) was adamantly opposed to it. Assuming that Juana's side wins the war thanks to the Portuguese Army having better luck or maybe more French involvement, perhaps it could be even possible to bring other countries to the game such as England or Granada, and not due to a greater amount of support from the Castillan nobility, their victory is a lot more akin to an outright Portuguese conquest of Spain than a simple resolution of a succession dispute (even among the aristocracy that fought on Juana's side, a very large portion of it was of Portuguese descent and had economic interests that were tied to those of Portugal). This means that Afonso would be way less hostage of the Castillan nobility than Ferdinand was IOTL. Some measure of appeasement will probably be necessary, since he probably can't kill or exile the entire Castillan aristocracy. But a large-scale restructuring of the Castillan ruling class is certain to happen, and this means that Afonso actually has a a chance of getting his way. If he cannot, well, I guess the only thing he can do is hope that Juana gives him a daughter that he can marry to the first grandson he has.
Now, the answers to your questions largely depend on the exact workings of the Union. How strong is the Castillan aristocracy? What is their composition? How tied are their intrests with those of the Portuguese? Does John II of Portugal inherit the throne of Castille? Without knowing the answer to these questions it's impossible to tell what the politics of this hypothetical union would be like.
a) What would be the official language today? Today in that area we have- Portuguese-Galician speakers 13 mil, Castillaian speakers about 23 mil.
Well, this assumes the union would last until the present day, which is a possibility, but let's not get too much ahead of ourselves.
If the union happens under the Portuguese Crown, Portuguese is likely going to be favoured in the court (I think it was still popular among Castillan elites of the time anyway...), but with time, demographics are surely on the side of Castillan.
That said the 15th century was a period when both languages were transitioning into their modern varieties, which means they were still closer to each other than they are today. The possibility of a different evolution cannot be discounted.
b) what would be the capital? Lisbon- because of the overseas trade and explorations or maybe Toledo or some other like Valladolid etc.?
If the Castillan aristocracy is strong enough, they are surely going to demand that the capital is in Castille. Otherwise, it will most surely be Lisbon.
c) colonial expansion? Would they reject Columbus as in OTL? Who would he work for? Wold they end up with whole OTL Latin America?
Yes, they would reject Columbus because they knew that his plan was stupid. Maybe Aragon funds his voyage (unlikely since, without Castille, they would most likely focus entirely in European power squabbles), or maybe it just doesn't happen. If it doesn't happen, John Cabot will discover America when he finds Newfoundland and the Portuguese will surely bump into Brazil soon enough.
If the get the whole of Latin America, that's impossible to know. If Portugal-Castille has a more "Portuguese" approach to colonization (likely if the aristocracy is not very strong), they are likely to eschew Imperial conquest in favour of trade, extortion and vassalisation of native peoples, which means it's unlikely they'll want to conquer the Aztec and Inca empires, though maybe the larger manpower causes them to be more like OTL Castille, frankly, I don't know.
d) What would be the relations between Portuguese and Castillians in that union? Who would have the upper hand? Would Portuguese want to separate like Catalonians today?
Once again, it depends on the power balance that is formed early on.
John II of Portugal as King of Castille would be any Castillan nobleman's worst nightmare. He was an autocratic King who severely curtailed the privileges of the Portuguese aristocracy IOTL, creating a strong, centralized state that had more in common with 17th and 18th century absolute monarchies than it had with the still largely feudal structures that predominated in European monarchies at the time. He was also a Portuguese nationalist and "renaissance man" who disliked the religious fervour and feudal-militaristic "Reconquista" mindset that traditionally predominated in Iberian nations (especially in Castille) at the time, and instead favoured rationalism, pragmatism and a colonial expansion strategy based on trade and the spreading of Western European culture. The only way the Union can even survive with him as king is if the Castillan aristocracy is really severely cucked.
If it does survive under these conditions, unlikely as it is, expect power to be centralized in Lisbon.
e) I presume that Moors in Grenade are toast by the POD?
Actually, since both Juana and the Portuguese monarchs tended to be way less militant with their catholicism than Isabella and Ferdinand, I'd say Granada is more likely to survive ITTL.
f) I presume that Portuguese will allow Castillians to settle in their colonies, being the smaller part of the union and lacking the manpower?
Hum... really depends on how the colonies develop in the first place. Lack of manpower was certainly a problem in the Portuguese Empire, but Portuguese traders tended to be very defensive when it came to keeping their monopoly.
