OK, this is the most effective, surefire way I've found so far:
1 - Open NTLite, mount the image you want language packs for. After it finishes mounting the image, you'll see something like 'Windows 10 Enterprise LTSC 2019 (1809) x64 - Client 17763.107' This last number is the build numbe and the information we need.
2 - Go to
https://uupdump.ml/ and look up your build number. A list of image links will show up, select one of the correct architecture.
3 - You'll be redirected to a language selection page, select the language whose packs you need, not the one from your base image. Click next, select 'all editions' from the list and click next again.
4 - Now click "Browse a list of all files" instead of any download options. You'll be redirected to a list with all the files on the image, the language packs are in there, somewhere. Click the names to download any you need, and before you leave, copy the section named 'rename script' at the end.
5 - Create a script (.bat or .cmd file) on the same directory of the files you downloaded, and run it to turn the unknown file types into proper cabinets.
6 - Now, go to your image directory and look up for the NetFX package, under Sources\SxS. You'll find a file named something like 'microsoft-windows-netfx3-ondemand-package~31bf3856ad364e35~amd64~~'. Copy that sufix after the first tilde, something like "~31bf3856ad364e35~amd64~~" and paste that at the end of the name of EVERY language pack you download.
7 - Move those to NTLITE install 'directory\updates\10.0.[your version number i.e. 1909].[Architecture i.e. x64].FOD'
And you're done. the Language Packs should now install without issues under NTLite. For some reason they are not activated under IME after the system is installed (M$ policies on needing you to connect to the mothership at least one, would be my guess), but they should integrate into the image successfully.