bluco

I’m sorry, my mistake. I didn’t read properly and I wasn’t uploading the languages files to /wp-content/languages/plugins/. I was uploading them to the own plugin language folder.

So, at least, let me summarize in case someone else have this issue in the future:

1) Download you language files (.mo and .po) from here: https://translate.wordpress.org/projects/wp-plugins/ultimate-member (I recommend stable version)

2) Rename the files properly. Spanish example:
ultimate-member-es_ES.mo
ultimate-member-es_ES.po

3) Upload them to /wp-content/languages/plugins/

4) Go to the folder /wp-content/languages/plugins/ and provide the right permissions and ownership like the rest of the language files. (chown and chmod)

And it’s done.