The LEGO Group seems to base it's mini-figures on stereotypes which are easily identifiable. I.e. you take one look at the mini-figure and you can usually tell which nationality, theme, genre, etc. it's supposed to represent/belong to.
The LEGO Group incorporated stereotypical Hispanic facial features into the Imperial Armada mini-figures and adorned them with stereotypical Spanish armament like morions and cuirasses which are usually associated with Conquistadors.
It seems rather confusing as to why they'd do this if they wanted to depict a different nationality.
Also the very name "Imperial Armada" suggest they're Spanish because Armada is a Spanish word. Obviously it's been adopted into the English language and could refer to a navy from another nationality but Spanish and Armada are rather synonymous. Being something else would only add to the confusion of the sub-theme's identity.