This post assumes you are familiar with creating MPQ archives and using Ladik's MPQ Editor, i won't make any detailed instructions, but merely go over the basics, Ladik's MPQ Editor can be found here: http://www.zezula.net/en/mpq/download.html
First of all, the following locales currently are supported by SC2:
1. Open the map you want to add locales for, .SC2Map files are just MPQ archives, so Ladik's MPQ Editor will have no trouble reading it.
2. Extract the map to a folder, for example c:\Maps\Mymap
3. Go into c:\Maps\Mymap and you will usually see 2 folders, and a bunch of files, depending on your locale it'll be either enGB.SC2Data, enUS.SC2Data or any of the other locales (xxXX.SC2Data), along with that there will be a "Base.SC2Data" folder.
4. Copy your locale folder (Example: enGB.SC2Data) and rename the copy to the locale you'd like to add (Example: enUS.SC2Data), personally i recommend doing this for all locales, there may also be french people who can speak english and would like english text over "param\blabla\blabla"
5. When you're done you must add it to the components file of the map, otherwise the editor will not pick it up (It will still work, but if you change anything in the map it'll delete it again), to do that you must open up "ComponentList.SC2Components" with an editor of your choice, it's just XML, so it's no big deal, anyway you have to add the correct locale to it, for reference here is the total list of locales you can add
6. Once you've added the locales you want to the componentlist, then you simply save it and open Ladik's MPQ Editor, you will select "Create new MPQ Archive"
7. Name of the archive will be the name of your map, just remember to mind duplicate names, so call it mymap.SC2Map
8. Select "Build the MPQ archive from a file or directory, and input the working directory you made earlier (c:\maps\mymap\)
9. Size of hash table should be set to atleast twice the amount of files you expect to have in the archive, however the default would work with most maps, as there aren't that many files.
10. Select to create the archive with the 2.0 format, and create (attributes)
11. You should now be able to open the file in your Map Editor, and verify that it works by opening the Overview Manager (F12), and verifying that the locales exist.
And you're done, of course the locales themselves will not be translated you will, however, be able to see the original english text in all clients now.