In JSTOR, it's fine to use keywords in your target language, because JSTOR searches the full text of every article within it.
JSTOR includes a significant number of scholarly ebooks in Spanish, as well as some in Portuguese and French.
JSTOR contains a significant number of articles in languages other than English, primarily Spanish and French.
In JSTOR, the best way to find articles in a target language is to use keywords in that language. JSTOR searches the complete text of every article it contains, so using Spanish keywords will likely find articles written in Spanish
However, you may also use JSTOR's Advanced Search screen, and select the Language limit.