Hi. !
In Europe, we have a many words in every language (French, Spanish, German...) that come from English, like bar and snack-bar. Except in England you have very difficult to find "fish & chips".
About word tuna as bar's name, I don't know which of the three meanings refer. But knowing a little about Portuguese cuisine, I guess refers to fish. Is widely used in Portugal.
"La Tuna" or "Estudiantina" is not exactly a group of traditional street musicians. Is a grouping or sorority of college students, wearing old clothes college, the image of the ancient goliards
Sagres in Portugal is ilke Bud Beer in Usa. Budweiser is a German adjective on the city of Budweis, the German name of Ceske Budejovice, a town in Bohemia, Czech Republic.
And, tuna as a Prickly Pear, is a Mexcican use of the word tuna and refer to nopal fruit, a different plant. In Spain, prickly pear is "higo chumbo" higo means fig, and chumbo not have any translation.
Hope this is useful.
Regards