1. Create new Resource files
2. Take .wsp language pack solution file for the SAME product and version (if no such version take the nearest version)
3. Change extension of language pack file from .wsp to .cab and open it, you'll see there:
folder with title of lcid number,
(see the image attached).
4. If the language will be the same and you modify LP just for new product version, then replace with new resources files in folders CONFIG\Resources AND Resources folders.
5. If you create LP for different language you have additionaly to do:
- rename folder with title of lcid number to the lcid of your language (if such folder is present),
- in manifest.xml file replace old lcid and change resource files' suffixes according to new LP language (if there is references in manifest file with lcid number).
6. Rename .cab file to .wsp.
7. Install created language pack, create a site and perform a smoke test to verify that translation is not corrupted.
In .cab archives you cannot change files/folders, so you have to extract necessary files, change them and copy into .cab archive back. And finally delete old unnecessary files/folders from .cab archive.
LCID numbers and language suffixes you can find in the following article,
see columns LCID Decimal and LCID string.
The list of possible language packs for SPLMS, REM, Dashboard is the same as the list of existing language packs for SharePoint, see the link.