You could try notepad ++, Im using it all the time since it is very userfriendly and it is free. Just open up the .xml files with it and you`ll see ^^

http://notepad-plus-plus.org/

In Elemental\data\English you'll find a file names english.str (str means "string")

Copy english.str in Elemental\Mods\data\English\ (you'll have to manually create the data and english folders)

Then edit english.str that lies in your mod folder.

Don't forget to enable mods in the game

Enjoy !

PS: it's a good thing to always work on copy of original files and put the copies in the mods folder with the same path as in the game root directory.

I have a new question. I have already translated all text from the file "Elemental.str" but I don't know what words should be translated into XML files.
I have tried to replace the words that are among <DisplayName> codes. For example:
 
 </ AIDifficultyLevel>
  InternalName="AIDiffLvl_Easy"> <AIDifficultyLevel
    <DisplayName> Fácil </ DisplayName>
    <AIEconomicRatio> 0.90 </ AIEconomicRatio>
    <AIFOWCheat> 0 </ AIFOWCheat>
    <AIHPRatio> 1.00 </ AIHPRatio>
    <AIStartingFundsRatio> 1.0 </ AIStartingFundsRatio>
    <AISovereignPointsRatio> 1.0 </ AISovereignPointsRatio>
    <AISovereignHPRatios> 1.0 </ AISovereignHPRatios>
    <AIIntelligenceFactor> 0.50 </ AIIntelligenceFactor>
    <AIUsesCombatMagic> 0 </ AIUsesCombatMagic>

However, this change is not reflected in the game. What am I doing wrong?

Thank you very much.

Changes to the Core files in the English directory would require to... recompile? the data.zip file, which is used instead of the XML files as a way to load faster the game. I'm not sure how it does work as I don't mod core files but some threads here talk about it (something about a command line to do it). Maybe one of the resident experts will post here about it.