$ ./isql localhost/3330:oltp30b -ch utf8
Database:  localhost/3330:oltp30b
-- проверяем чарсет коннекта:
SQL> set list on; select r.rdb$character_set_name from rdb$character_sets r 
CON> join  mon$attachments a on r.rdb$character_set_id = a.mon$character_set_id
CON> where mon$attachment_id=current_connection; set list off;

RDB$CHARACTER_SET_NAME          UTF8  

SQL> create domain dm_1 int; commit;
SQL> comment on domain dm_1 is 'йцукенг';
SQL> commit;
SQL> set blob all;
SQL> select rdb$description from rdb$fields where rdb$field_name='DM_1';

  RDB$DESCRIPTION
=================
              0:6
==============================================================================
RDB$DESCRIPTION:
йцукенг
==============================================================================

Server version:
Firebird/Linux/AMD/Intel/x64 (access method), version "LI-T3.0.0.31309 Firebird 3.0 Alpha 2"
Firebird/Linux/AMD/Intel/x64 (remote server), version "LI-T3.0.0.31309 Firebird 3.0 Alpha 2/tcp (oel64)/P13"
Firebird/Linux/AMD/Intel/x64 (remote interface), version "LI-T3.0.0.31309 Firebird 3.0 Alpha 2/tcp (oel64)/P13"
on disk structure version 12.0
SQL> show database;
<...>
Default Character set: NONE

C:\>isql -ch utf8 192.168.0.220/3330:oltp30b
Database:  192.168.0.220/3330:oltp30b
SQL> show version;
ISQL Version:  WI- V2.5.3.26730 Firebird 2.5
...
SQL> set list on; select r.rdb$character_set_name from rdb$character_sets r
CON> join  mon$attachments a on r.rdb$character_set_id = a.mon$character_set_id
CON> where mon$attachment_id=current_connection; set list off;

RDB$CHARACTER_SET_NAME          UTF8

SQL> create domain dm_2 int;
SQL> commit;
SQL> comment on domain dm_2 is 'йцукенг';
Statement failed, SQLSTATE = 22000
Dynamic SQL Error
-SQL error code = -104
-Malformed string