translate Milestone

You can suggest translations for the Jetpack plugin (and all the other plugins) via translate.wordpress.org:
https://translate.wordpress.org/projects/wp-plugins/jetpack

It looks like it was already translated into Faltan %1$s %2$s. in Spanish (Spain):
https://translate.wordpress.org/projects/wp-plugins/jetpack/stable/es/default?filters%5Bstatus%5D=either&filters%5Boriginal_id%5D=3920110&filters%5Btranslation_id%5D=47118318

If you don’t see this translation on your site, try to go to Dashboard > Updates and update translations to download all freshly translated strings on your site.

If your site uses a different version of Spanish, you’ll need to look for the translation in that language, and suggest a new translation if necessary.

I hope this helps!