Game when installed is around 64GB, but the manual install files are around 110GB.

How come? Installation files are usually the same or smaller than the final install, so why the opposite here?

Only thing I can think of is it contains extra data for languages that aren't installed, if so why not give the option to only download install files for the language you want?

Only ask as I'm backing up all my GOG install files and my crappy internet (8MB/s DL) take ages for just this one game alone and downloading effectively 40GB extra for nothing is a huge waste of time (and GOGs server resources).

Yes, that's it.

If you are really low on space and still want to keep the offline installer you can try to write down which of the files are read when installing in your selected language. The installer definitely skips a number of those 4GB files while installing (if you also skip the checksum at the start). Then you could delete the unused installer files and still later use the remaining files to install the same language.