Once you are sure the data is in Unicode, it is important to create a list of all the alphabetic characters in the vernacular language. This will show if there are characters that don't belong in the data, and it will be helpful for setting up the "sort" order in FLEx. The list is needed for publishing on the Webonary Alphabet page. Sometimes two different code points have been used for the same alphabetic character. This will need to be fixed for the "sort" order and letter headers to show up correctly.
This can be done with the Unicode Character Count Utility Since this is a Windows .exe file, you'll need to run this in a DOS command box, not in WSL. Be aware that the .exe creates UTF-16LE with a BOM mark at the beginning of the file. You can convert the file to UTF-8 with the WSL dos2unix command.
A perl script version of the utility is available in a zip archive at the same URL. You can run the perl script in WSL.
If the analysis language is an uncommon language in FLEx, it may be necessary to run this utility for it as well.
The WSL uniname utility is useful for identifying unknown Unicode characters. It dumps all the characters in a file, one per line, with details of their Unicode names. Install it with this command:
sudo apt install uniutils
A gui tool to identify characters in a string is available from Keyman:
https://downloads.keyman.com/tools/charident/charident.exe