I don't think the structure is quite right. messages table doesn't need a text column and multi language is not considered. Perhaps this

message_groups

• id int(10) unsigned NOT NULL AUTO_INCREMENT,
• action varchar(64) NOT NULL,
• type ENUM('client', 'staff', 'shared') DEFAULT 'client',
• plugin_dir varchar(64) NOT NULL
• PRIMARY KEY (`id`),
• INDEX `type` (`type`)

messages

• id int(10) unsigned NOT NULL AUTO_INCREMENT,
• message_group_id int(10) unsigned NOT NULL,
• company_id int(10) unsigned NOT NULL,
• type ENUM('sms') DEFAULT 'sms',
• PRIMARY KEY (`id`),
• INDEX `message_group_id` (`message_group_id`)

message_content

• message_id int(10) unsigned NOT NULL,
• lang VARCHAR(5) NOT NULL
• content mediumtext COLLATE utf8_unicode_ci
• PRIMARY KEY (`message_id`, `lang`)

I think we could add a "name" column to the "messages" table, which will be shown in the "Message Templates" view.

Perhaps these tables could have better names. perhaps message_groups => messages, message => message_types. Would a name field serve any other purpose? It's a little hard to place because the current messages table is associated with a type so having a name for every type doesn't make much sense, but message groups are not distinguished by company so it can't go there either.

Maybe we could use the "action" of the group, to define the name using a language definition, to not add another column to the table.

$this->_('AdminCompanyMessengers.templates.message_group_' . $group->action);

I like that

Another thing, if the messages are going to support tags, we would have to store them somewhere, maybe it could be in "message_groups" similar to "email_groups".

Agreed, and it should be on the message edit page like we do for emails