Currently, there's wp language core activate and --activate flags for the other commands.

Activating a language is nothing more than wp option update WPLANG <locale> and checking whether that language actually exists.

In #21, I started working on subcommands for plugins and themes. Activating a language is a global action per site. One cannot activate language X for plugin A and language Y for plugin B.

Should wp language core activate be renamed to wp language activate or wp language switch? See #21 (comment)