You can use the windows api function SHGetFolderPathW with folder code (csidl) 5.

In PFCs platform service you can find the function of_GetKnownFolderPath as a codes example how to use the function.

https://github.com/OpenSourcePFCLibraries/2022/blob/main/ws_objects/pfcapsrv/pfcapsrv.pbl.src/pfc_n_cst_platformunicode.sru

Get the users' directory first with GetContextKeywords: 

ls_envkey = "%userprofile%"

or

ls_envkey = "%HOMEPATH%"

ContextKeyword icntext_keyword

icntext_keyword.GetContextKeywords(ls_envkey, ls_values[])

then append the subdirectory you want to use:

ls_values[1] + "\Documents"

 which is exactly what you don't want to do since it is locale dependent. 

It is weird that windows has all of these %values% but not one for documents.  

 windows - Is There A System Defined Environment Variable For Documents Directory? - Stack Overflow