You can chain REPLACE functions:
select replace(replace('hello world','world','earth'),'hello','hi')
This will print hi earth
.
You can even use subqueries to replace multiple strings!
select replace(london_english,'hello','hi') as warwickshire_english
from (
select replace('hello world','world','earth') as london_english
) sub
Or use a JOIN to replace them:
select group_concat(newword separator ' ')
from (
select 'hello' as oldword
union all
select 'world'
) orig
inner join (
select 'hello' as oldword, 'hi' as newword
union all
select 'world', 'earth'
) trans on orig.oldword = trans.oldword
I'll leave translation using common table expressions as an exercise for the reader ;)