"an URL parameter 'hl=es' will set the language of the site to Spanish".   How to use the 'hl' parameter? What is the syntax?

https://your-web-site.tdl/?hl=es

"If you wish to help translating Elgg..."   I have already registered for transiflex but it seems that someone at Elgg must approve it. 

I would also like to add a new language: Esperanto (eo). I have done a test by creating 'eo.php' in my site and the new language is recognized correctly.

approved the EO language

Elgg says that the Spanish language is 52% translated, but comparing 'es.php' with 'en.php' I find that many lines are missing in 'es.php'.

As you would expect with only 52%

It would be good to add this information to the Developer Guides:Internationalization (http://learn.elgg.org/en/stable/guides/i18n.html) where it only deals with the translation of the plugins and does not mention the main language files.

That is because the main languages files are managed by Transifex. We pull the translation before every release of Elgg.

It's not recommended to manually add language files to the Elgg core and Elgg core plugins as when you update those files might get lost or overridden.

approved the EO language

Thank you. I cannot translate because I am not a member of the Esperanto team. That's what Transifex says

Regarding the request to add Spanish (Argentina) to the project I never did it. (at least intentionally)

Thank you. I cannot translate because I am not a member of the Esperanto team. That's what Transifex says

DM me your Transifex username/email and I'll add you. And if you wish I can add you to Spanish as well, let me know in the DM.