Internationalizing makes a plugin’s GUI appear in the native language of the user.

Internationalization process of 3P plug-ins⁠

A third-party can only internationalize “new style” Scheme plugins installed to /plugins.

The process is well documented at Internationalizing Plug-Ins

Internationalizing of GIMP own plug-ins⁠

As said, it is not possible to localize third-party old-style scripts installed to /scripts. Only GIMP own scripts can be localized.

That is because the extension-script-fu plugin has its own i18n declaration that is used for all served scripts. So served scripts all use the domain gimp30-script-fu and the only one corresponding translation data installed with GIMP, which is /usr/local/share/locale/es/LC_MESSAGES/gimp30-script-fu.mo.

See the use of the macro DEFINE_STD_SET_I18N in the source files script-fu.c and script-fu-intl.h if you are curious.