PolyLang WP Update to replace one string to another
Basically, you constuction to get the right ID was doubles and UPDAT has no fROM clause
There WHERE clause you should try first in a SELECT if you get all IDs
UPDATE wp_posts 
SET post_content = REPLACE(post_content, 'str1', 'str2')  
WHERE ID IN (SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(description, '&quot;ar&quot;;i:', -1), ';',1) FROM wp_term_taxonomy WHERE taxonomy = 'post_translations')