You must always install language packs on servers with the application role. Language packs are site specific, that is, the list of language packs that the Messaging system displays in User Preferences for a user is dependent on the language packs installed on the application server serving that site.
If there is a cluster of application servers for a specific site, then the Messaging system retrieves the list of language packs that you installed from the first application server. Hence, you need to ensure that you install the same set of language packs on all cluster members.
Messaging supports users who are hearing impaired or speech impaired. Such users can transmit and receive text using a teletypewriter (TTY) device. You must install the English (United States) - TTY language pack to use the TTY functionality.