When you run WordPress in Multisite Mode, you can enable WPML as a network plugin or per individual child blog.
For network activation, go to Network Admin → Plugins. There, click on Network activate for WPML.
Once activated for the network, child blogs become multilingual. You need to set up languages for each child blog. This includes WPML’s setup wizard. Each child blog can have different languages.
WPML’s Troubleshooting page adapts to network mode. When doing network activation, you can reset WPML for individual blogs from Network Admin → WPML → Network settings.
Just like any other network plugins, in multisite mode, when WPML is network-activated, it cannot be activated or deactivated individually for child blogs.