You can use client command line options to say the client which locales to use.
In case you are using the Ryzom Installer, you can set it in a profile (I am not sure if the new installer is on ryzom web, however the new version is at https://download.ryzom.com/). You can also pass command line options in Argument field of a profile.
Other option is to set the language manually in the client config file (client.cfg). Add / find and change the following line. Use two-letter codes along ISO 639-1.
In case you are using the Ryzom Installer, you can set it in a profile (I am not sure if the new installer is on ryzom web, however the new version is at https://download.ryzom.com/). You can also pass command line options in Argument field of a profile.
Other option is to set the language manually in the client config file (client.cfg). Add / find and change the following line. Use two-letter codes along ISO 639-1.
Example
LanguageCode="en";