I am using JMS 1.2.69 on a Joomla 1.7.3 site with 2 languages installed. Everything works fine on the master site but I cannot find the way how to enable the same languages on the slave sites. I cannot find the option in the site management to share the languages and if I try to install the language pack on the slave sites it comes back with an error message that the plugin does not exist on the master site.

Go in the "discover" option present in the "extension manager" of the slave site back-end.

Joomla 1.7 has changed the management of the languages and templates that are no more automatically detected by joomla but required to be "discovered" manually.
So you have to use the "discover" functionality present in the Joomla "extension manager" menu.

Thanks Edwin, it has solved my problem.
I am just testing this environment so I was not aware of this new feature of Joomla 1.7. There was an idea coming to my mind reading the description of "discover". Is it possible to install any other extension from the master site to the slave sites with this method? I have problems with the extensions of NoNumber because of the special installation process of his extensions. I have the workaround how to do it but it is quite complicated. I will give a try.

No number has a special installation that use a temporary component to install the final one. This is the reason why "discover" does not work.

For the extension that are defined in the JMS Tool, you can install the "no number" in a slave site.