Capability has been implemented in the API database connection (DPB) area to interoperate with the character set and/or code page of server and client, to avoid the previous problems that could occur when file names contained non-ASCII characters.

Refer to the topic Connection Strings & Character Sets in the chapter Changes to the Firebird API and ODS. Even if you are not normally interested in the API, this topic will be a worthwhile read if you have been bothered with such issues.