If you want to keep your elgg page strictly in one language, you don´t want to give your users the option to change it. So, just disable the language settings from the users settings menu.
Here is how to disable the language settings in elgg:
- On your elgg installation, change to the directory
change <elgg installation> with the path you installed elgg into.
- Rename the file language.php to something else:
mv language.php language_old.php
That is it, no need to restart anything, The next time the user switches into his settings page, the the language settings area is completly gone.