Hi I've read in the collections docs in the section about stemming this:

Language selection for stemmer is automatically made from the value of the locale property associated with the field.

then in the locale section there is a list of locale and this note:

For all other languages, you don't have to set the locale field.

If I want to get spanish stemming if I set

stem=true

and

locale=es

it would work?

Yeah, it should work