add include localized file token

doc/file/format.txt

@@ -33,6 +33,13 @@ A heirachical file can contain a reference to another file:
 Where filename must be an absolute or relative [[type:filename]].
 
 That file is included literally into the current one; except for indentation, the included file never escapes from the level the 'include file' line is on.
+
+If the file to be included can vary depending on the locale that is selected, use:
+>>>include localized file: <em>filename</em>
+
+MSE will take the filename and add "_" followed by the name of the currently selected locale at the end of it.
+So for example, if the locale used is the folder "en.mse-locale", the file that will be included is "filename_en"
+You must provide a version of the file for each locale found in the data folder, even if it is simply a copy of the english one.
 
 --Example--
 For example, a [[type:set]] might look like this: