MySQL 8.0

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CREATE TABLE table1(
   id         VARCHAR(100) NOT NULL 
  ,id_related VARCHAR(100) NOT NULL

);
INSERT INTO table1
(id,id_related) VALUES 
('id1','[id2,id3,id4]'),
('id2','[id1,id4]'),
('id3','[id1,id2,id5]');

CREATE TABLE table2(
   id      VARCHAR(100) NOT NULL 
  ,country VARCHAR(100) NOT NULL
);
INSERT INTO table2(id,country) VALUES 
('id1','BR'),  
('id2','US'),
('id3','CO'),
('id4','IT'),
('id5','US');
SELECT  id,TRIM(BOTH '[' FROM  TRIM(BOTH ']' FROM id_related)) as id_related
FROM table1


id
id_related


id1
id2,id3,id4

id2
id1,id4

id3
id1,id2,id5


select t12.id,t12.id_related from
(select
t11.id,
SUBSTRING_INDEX(SUBSTRING_INDEX(t11.id_related, ',', numbers.n), ',', -1) id_related
from
(select 1 n union all
select 2 union all select 3 union all
select 4 union all select 5) numbers INNER JOIN 
(SELECT  id,TRIM(BOTH '[' FROM  TRIM(BOTH ']' FROM id_related)) as id_related
FROM table1) t11
on CHAR_LENGTH(t11.id_related)
     -CHAR_LENGTH(REPLACE(t11.id_related, ',', ''))>=numbers.n-1 ) t12
    


id
id_related


id3
id1

id2
id1

id1
id2

id3
id2

id2
id4

id1
id3

id3
id5

id1
id4


select t12.id,t13.id_related,concat('[',GROUP_CONCAT(t2.country ORDER BY t12.id_related asc ),']') AS  related_country  from

(select
t11.id,
SUBSTRING_INDEX(SUBSTRING_INDEX(t11.id_related, ',', numbers.n), ',', -1) id_related
from
(select 1 n union all
select 2 union all select 3 union all
select 4 union all select 5) numbers INNER JOIN 
(SELECT  id,TRIM(BOTH '[' FROM  TRIM(BOTH ']' FROM id_related)) as id_related
FROM table1) t11
on CHAR_LENGTH(t11.id_related)
     -CHAR_LENGTH(REPLACE(t11.id_related, ',', ''))>=numbers.n-1 )
     t12
     join table2 t2
     on t2.id=t12.id_related
     join table1 t13
     on t13.id=t12.id
     group by t12.id,t13.id_related


id
id_related
related_country


id1
[id2,id3,id4]
[US,CO,IT]

id2
[id1,id4]
[BR,IT]

id3
[id1,id2,id5]
[BR,US,US]
id	id_related	related_country
id1	[id2,id3,id4]	[US,CO,IT]
id2	[id1,id4]	[BR,IT]
id3	[id1,id2,id5]	[BR,US,US]