hi,
TVH is by default getting the encoding from over the air satellite transmission of DVB stream which sets appropriate decoding which in some cases is wrong just like it was for polish platforms or Sky DE. It has been fixed since then for them as TVH is capable of overriding the provider settings which are not always right by using the charset file.
https://www.lonelycoder.com/redmine/projects/tvheadend/repository/revisions/master/entry/data/conf/charset
I could prepare the update for this listing but would need to know which ISO standard is appropriate for You and which platform do You have wrongly set up by TVH, as it may be wrong just for one of them and seems that for instance FEM 3 channel is aired for Magio, Digi & UPC http://en.kingofsat.net/find.php?question=fem3&Submit=Zap
In the mean time you can try to override it for specific channels "Configuration -> TV Adapters -> Services" Tab under "DVb Charset" setting
To test if the override works you need to stop TVH, remove the EPG DB and see if it is correctly shown after you start it again