Message data which is intended to be displayed according to the locale of a client.A `LocalizableMessage`_ contains both a formatted, localized version of the text and the data needed to perform localization in conjunction with localization catalogs.Clients of the VIM API may use vim.SessionManager.setLocale() to cause the server to emit a localized `message`_ , or may perform client-side localization based on message catalogs provided by the server.
* If the substition variable is a string, no further lookup is required.
*
*`arg`_
* = [("address" = "127.0.0.1")]
* CATALOG(locmsg,
*`key`_
* ) = "IP address is {address}"
* ==
*
*`message`_
* = "IP address is 127.0.0.1"
* If the substitution variable is an integer, value is a lookup key.