Firebird 2.5 UTF8 database, WIN1250 client problem (firebird bug?)

INSERT INTO sz_felirat_szall (id, nyelv_kod, mezonev, ertek)
VALUES (2065664, '00', 'LEV_SKOMP', 'Szállítói tartozásunk kiegyenlítése')

CREATE TABLE SZ_FELIRAT (
    NYELV_KOD         XVAR2N /* XVAR2N = VARCHAR(2) NOT NULL */,
    ...
    LEV_SKOMP         XVAR40 /* XVAR40 = VARCHAR(40) */,
    ...
);

CREATE TABLE SZ_FELIRAT_SZALL (
    ID         XIDN /* XIDN = INTEGER NOT NULL */,
    NYELV_KOD  XVAR2N DEFAULT '00' /* XVAR2N = VARCHAR(2) NOT NULL */,
    MEZONEV    XVAR30N /* XVAR30N = VARCHAR(30) NOT NULL */,
    ERTEK      XVAR100N /* XVAR100N = VARCHAR(100) NOT NULL */
);

CREATE OR ALTER TRIGGER insertsz_felirat_szall FOR sz_felirat_szall
ACTIVE BEFORE INSERT POSITION 0
AS
DECLARE VARIABLE ins VARCHAR(300);
DECLARE VARIABLE db INTEGER DEFAULT 0;
BEGIN
  IF (NEW.id IS NULL) THEN NEW.id = GEN_ID(xid_gen, 1);
  IF (NEW.nyelv_kod IS NULL) THEN NEW.nyelv_kod = '00';
  SELECT
      COUNT(1)
  FROM sz_felirat
  WHERE nyelv_kod = NEW.nyelv_kod
  INTO :db;
  IF (db = 0) THEN INSERT INTO sz_felirat (nyelv_kod)
    VALUES (NEW.nyelv_kod);
  ins = 'update sz_felirat set ' || NEW.mezonev || '=''' || REPLACE(NEW.ertek, '''', '''''') || ''' where nyelv_kod = ''' || NEW.nyelv_kod || ''' and coalesce(' || NEW.mezonev || ', '''') = ''''';
  EXECUTE STATEMENT ins;
END