Default Unpack system.img, add system language and repack it

Hi there,
I have a chinese phone Haier W718, as I buy it to make ROM modifications and learn how to compile and edit Android ROMs. I have the system.img dump from the original ROM and now I want to make the following.
1. Decompile the system.img file
2. Add new system languages
3. Compile and flash the ROM
So my question is how to properly decompile it and add the language that I want? I search for some tools in XDA but I am not sure if the things that I found are the proper ones. Can you please give me the right links for tools about this whole process. I also want to find some more information how to properly add a system language. Thank you in advance! By the way, I am using Linux so it will be better if the tools for compile and decompile are Linux compatible.
Cheers