Works for me:
~ % cd /tmp
/tmp % mkdir source target
/tmp % cd source
source % ~/go/bin/duplicacy init test /tmp/target
/tmp/source will be backed up to /tmp/target with id test
source % echo hello > åäö.txt
source % ls
åäö.txt
source % ~/go/bin/duplicacy backup
Storage set to /tmp/target
No previous backup found
Indexing /tmp/source
Parsing filter file /tmp/source/.duplicacy/filters
Loaded 0 include/exclude pattern(s)
Packed åäö.txt (6)
Backup for /tmp/source at revision 1 completed
source % ~/go/bin/duplicacy list
Storage set to /tmp/target
Snapshot test revision 1 created at 2021-08-12 15:11 -hash
source % ~/go/bin/duplicacy list -r 1 -files
Storage set to /tmp/target
Snapshot test revision 1 created at 2021-08-12 15:11 -hash
Files: 1
6 2021-08-12 15:10:57 93becc6e9882211c3ec3708c95bcd69baab7bb59c7f4bc84ce637b88a534b783 åäö.txt
Files: 1, total size: 6, file chunks: 1, metadata chunks: 3
source %
So duplicacy seems to handle the UTF-8 correctly, and the issue is somewhere else ( I have a metric ton of files named in Cyrillic myself, so that was never an issue)
What do you have in your filters file?
You can run backup with -d
global flag to see why the files are not being picked up.