The encoding you use in your code is the key.
For French or German, you can use either UTF-8 or ISO-8859-1. both will work. I prefer to use UTF-8 since it supports all other languages too.
when you hanle string in the flow service, consistently use UTF-8 (or the other one you choose).
When you retrieve data from DB, make sure you know the encoding it’s in, if needed, you may write some java code to convert it to UTF-8 first.
HTH,
Tong