Oracle 18c

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CREATE TABLE t(country_code VARCHAR2(3))
BEGIN
 INSERT INTO t VALUES('GB');
 INSERT INTO t VALUES('CH');
END;
/
1 rows affected

WITH
  FUNCTION with_function(i_countries VARCHAR2) RETURN VARCHAR2 IS
    o_val VARCHAR2(10);
  BEGIN
     SELECT CASE WHEN SUM(NVL2(t.country_code,0,1))=0 THEN 'false' 
                 ELSE 'true'
                  END
       INTO o_val           
       FROM (SELECT DISTINCT REGEXP_SUBSTR(i_countries,'[^ ,]+',1,level) AS country
               FROM dual
            CONNECT BY level <= REGEXP_COUNT(i_countries,',')+1) tt
       FULL JOIN t
              ON tt.country = t.country_code;      
    RETURN o_val;
  END;
SELECT with_function('GB,CH,TR') AS result
  FROM dual


RESULT


true