Hi Juan,

Its possible to create new Locales just go to the web keycloak admin console >select your realm>settings>Themes tab

Here you can enable internationalization and type a Locale(if already not present in dropdown) and hit enter.

Then you will have to create a property file for the messages(that will contain the text in the new language).I am giving an example for login page but you can use the same every where.

1. open folder "KEYCLOAK_HOME/standalone/configuration/themes/base/login/messages" 
   
2. create a file  messages_NEWLOCALE.properties and enter the messages in the translated language.
   

You can use the existing properties files as reference. 
Here KEYCLOAK_HOME is the location where keycloak is located and NEWLOCALE should be the name of the local you have created.

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Best Regards