Sort of, but not quite.

- create a new folder for your mod in:
C:\Users\YourUserName\AppData\Local\Introversion\Prison Architect\mods

- call it:
mods_UK_Prisons

- create a manifest.txt in this folder, with content:
note: The 'Name' is like the folder name, the space between UK and Prisons is resembled by the underscore in the mod foldername. You should adjust the date and version number when you update your mod. Since you want to alter the language and English is default anyway, I think you can leave 'IsTranslation' to false instead of true.

- create a thumbnail.png so the mod has an image to show in the workshop
- create a subfolder called data
- create a subfolder called data\language
- create a base-language.txt in that folder and copy the lines which you want to change from the default language file into your fresh language file.
- create a d11.txt as well, if you need to change tetx from the new stuff which was added.
- activate the mod in your mods list to see if it works, there you can also publish it.

Thank you very much Murgh!
I was just poking round the game file and trying to hazard a guess. Such a great detailed tutorial comment. Thank you again!

I'll have to have a play around this afternoon and do some testing. :)

Awesome! I had a spare 20 mins and did some testing. Got it working in game. Thank you so so much!

Now just to go through that original language file with a fine tooth comb. Feel like this is gonna take a while lol.