adding an additional spelling dictionary problems (10.14.6)

I'm trying to add a new spelling dictionary (Greek) to my system so to be able to do a spell check in Greek from Pages. I have the 2 files needed (Greek.dic and Greek.aff) but I have no idea where to put them. In System -> preferences -> Keyboard -> Text, when I choose set-up in spelling, it says:

Well, there is no spelling folder inside /Library. After some searching I found that in Mojave this folder is the:

/System/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionaryOSX

folder.

The problem is I cannot change the permissions to that folder as it's read only and its owned by system. When I'm trying to add my user to the sharing and permissions area it stops me and says:

Any help to that?

1. Is this the correct folder?
2. How do I change the permissions?

Thanks in advance.