You should make a video on this it will be WAAAAAY easier >.<

So we don't rename the .fsb file at all? Can't seem to get the in-game VO to work

I understand all other steps other than:

Find at the top where it says “LOL_Audio” in the new language and change it so it matches the filename. You’ve effectively told it to check within itself for the addresses.

Please help if you can. Thank-you.

^
What that means is that within your text editor (in Notepad++ it's on Line 3), let's say you're editing the Italian sound files to be in...uh, French, because you live in France? Yeah. That's it.
[shh just roll with it for this example]

Doing a search pulls up:
LoL_Audio_it_IT

change the "it_IT" there to the language your client's domestic language IS IN. replace those 5 characters with "fr_FR". Do so by hand, because a search query will mess up the rest of the file.

Also since some people seem to be having issues with the rest of the client editing (i'm by far the worst video editor, a video so wouldn't help) to clarify, using the same Italian to French example and a lot of linebreaks to make things simpler.

Changing the voices from Italian to French once you have the .fev open is easy.

Search and replace (CTRL+H) "VOBank_it_IT" and request that it be replaced with "VOBank_fr_FR".
This replaces instances of the ENTIRE STRING. Simply doing "it_IT" to "fr_FR" will DAMAGE the files.

Each .FEV contains directory paths for that respective language, to have the audio file successfully work.

EXAMPLE: unitvo/akali/Italian/it_IT_Akali.effort2.wav

Changing the "it_IT" string ONLY in the above example would cause ALL "it_IT" strings to change, including these directory paths, resulting in
unitvo/akali/Italian/fr_FR_Akali.effort2.wav

and that would break the filepath, resulting in sounds that don't work.

so MAKE SURE YOU SEARCH AND REPLACE THE ENTIRE "VOBank_##_##" STRING!

EXCEPTIONS:
Chinese region zh_CN client's filepaths resemble American ones. No variation in directories.
EXAMPLE FILEPATH, FROM LOL_Audio_zh_CN: unitvo/akali/Akali.effort2.wav
So, you can be lazy and simply run a "zh_CN" -> "en_US" search replacement query, but it is not recommended for consistency purposes.

ALSO FOR THE RECORD:
.fev = FMOD Designer Project files. This file tells where to look in the FMOD Sound Bank (as well as the actual game files) for voice clips and sound effects.
.fsb = FMOD Sound Bank file. Think of this as a big ZIP file but for videogame sounds.
These soundbanks are used in other PC games such as Just Cause and Crysis.

I don't think I could clarify any clearer than that.
I don't wanna be a jerk, but if it still isn't working out, I'd just hold off until a more streamlined solution arises, such as a program that automatically changes them on the fly a la Dargon [which I cannot code to save my life sooooo] @_@

Does this still work as of this patch?

Nevermind. I think i found a better way to do it

How do you get the chinese files?
Edit: to clarify, do i need a proxy?

Ok I exactly went through all the steps by using your tutorial, its really well written, btw! No missunderstandings so far I got one problem, though.. After I completed the final step and launch the game, the launcher starts patching and overwrites all the work we did.. Has it something to do with the files, I replaced in the FMOD folder?

I got both the korean sound files and well now im i notepad++ and i changed all the VOBank_ko_KR into VOBank_en_US. And I also changed the LOL_Audio_ko_KR into LOL_Audio_ko_KR. However when i press league it states that it failed to have a translation in start up