All Plex c++ client applications having national language problems. They have been working fine so far. Most likely new win 10 update damaged push buttons with national language characters. WIN 7 machines work fine.
On Windows 10, modified the Region settings.
Only required modification was to Change Region /Format tab/Finnish(Finland) which Win 10 uppgrade had changed to English(Finland)
Region /Format tab/Finnish(Finland) in our not upgraded Win 10 machines.