If INCLUDE_LANGUAGES is defined inside installer-brand.nsi, then all available langage files (*.lng) will be included into installer.
If you do not want to include all languages, then just commend this line:
!define INCLUDE_LANGUAGES
In the case you want to include only single language into the installer (and skip all others), then use option INCLUDE_SINGLE_LANGUAGE, which specifies the name of language file, which will be solely included into installer:
!define INCLUDE_SINGLE_LANGUAGE "Spanish.lng"