You don't need to edit the English.str file to accompish that. Say you want a militia on your map to have a certain name. All you have to do is define an additional player, name them what you want, and so long as isNormalPlayer, isRaidingPlayer, and isInsurgentPlayer are all set to FALSE, the player will act just like a normal militia. If you do that however, I don't think the OwnedByMilitia and OwnedByMilitiaWithChance calls will work for template. I've been using the Always condition and then setting the owner to whatever name I defined for the custom militia. However, I haven't tried using those conditional calls so I cloud be oncorrect on that point.

For Example:

player
  	designName "Smurf Militia"
  	inGameName "Smurf Militia"
  	overrideRaceName "Tech"
  	teamIndex -1
  	startingCredits 3000
  	startingMetal 800
  	startingCrystal 250
  	isNormalPlayer FALSE
  	isRaidingPlayer FALSE
  	isInsurgentPlayer FALSE
  	themeGroup "NPC"
  	themeIndex 2
  	pictureGroup "NPC"
  	pictureIndex 2

Question on a related note:

Can I rename the pirates using the method above? Could I define the following:

player
  	designName "Smurf Pirates"
  	inGameName "Smurf Pirates"
  	overrideRaceName "Pirate"
  	teamIndex -1
  	startingCredits 3000
  	startingMetal 800
  	startingCrystal 250
  	isNormalPlayer FALSE
  	isRaidingPlayer TRUE
  	isInsurgentPlayer FALSE
  	themeGroup "NPCSpecial"
    	themeIndex 2
  	pictureGroup "NPC"
  	pictureIndex 2

If I set up a pirate base on my map and assign this player as the owner, will this effectively rename the pirates?

I tested this today and it didn't result in active pirates. I set up a player as above then then assigned the pirate base world to the owner "Smuf Pirates". The pirates showed dead as soon as the map loaded.