u/xymmachus is right that there was a lot of contact between the Armenians and France due to the crusades, but Mendo Alão is supposed to have lived long before that. It’s true that (as xymmachus) mentioned) there were marriages between French and Russian nobility in the 11th century, and Western Europeans often married Byzantine princesses as well, so diplomatic ties between western and eastern Europe were strong. But were there any relationships between Europe and states even further east?

This princess would supposedly have been from the Artsruni dynasty (the name of her family, not her own name). That was a Bagratid Armenian dynasty, closely related to the rulers of Georgia. At the time Armenia was in the far east of Anatolia and the Caucasus. Just to reinforce what xymmachus has already mentioned, this is not the same Armenia that was allied with the crusaders, which was “Cilician” Armenia, in southern Anatolia. Of course, the Rupenid rulers of Cilicia were (probably) descended from the Georgian-Arrmenian Bagratids, but still, they’re pretty different. How would the Bagratid Artsuni dynasty have any contact with Brittany?

As for “Alain of Nantes”, the County of Nantes wasn’t usually an independent county, but was a title held by the Duke of Brittany (as Nantes was one of the major cities of Brittany, along with Rennes). There are lots of counts of Nantes and Dukes of Brittany named Alain. The only ones from around the same time as Mendo are:

- Alain Barbetorte, or Duke Alain II of Brittany, who died in 952, too early for this story;

- Alain II’s grandson Alain, the only possible Alain according to all the info I can see (Mendo’s father always seems to be a “grandson of Duke Alain II”). But he died as a child in 990!

- Duke Alain III, who was duke around the right time (1008-1040), but he was never Count of Nantes and his wife and family are well known (he was married to the daughter of the Count of Blois)

- Duke Alain IV, who was Count of Nantes, and who participated in the First Crusade, but he wasn’t married to any Armenian princess, and he’s too late for this story anyway.

So, because this story is several generations too early for any contact between Bretons and Armenians; there is no possible Alain of Nantes who could be his father; and Artsruni is the name of an Armenian dynasty, not the name of the princess; it seems to me that this is a completely fictional genealogy. Mendo is supposedly an ancestor of the Braganza dynasty, which eventually became the royal dynasty of Portugal, so maybe they wanted to have a more fanciful, exotic ancestry than their real origins - they descended from Duke Afonso of Braganza, the illegitimate son of King John I of Portugal.

Also, I hope this doesn't come across as a crazed rant, but it’s important to remember that genealogists are nothing if not extremely credulous. (Early genealogists, at least. Modern genealogists are better! Well, sometimes…) Genealogies are more useful for finding out what people believed, regardless of whether it was true or not.

Hopefully a specialist in Portuguese history could help more, but I feel confident in saying the Breton-Armenian parts of this story are fictional.

I have some sources for the history of Brittany during this period, if that helps (they’re in French though so maybe not):

André Chédeville and Noël-Yves Tonnerre, La Bretagne féodale, XIe-XIIIe siècle (1987)

Noël-Yves Tonnerre, Naissance de la Bretagne: Géographie historique et structures sociales de la Bretagne méridionale (Nantais et Vannetais) de la fin du VIIIe à la fin du XIIe siècle (1994)