darkoP-9

Problem:
Translated versions of pages were redirecting to the default language for all the pages and posts.

Solution:
After detailed investigation, I found that "SEOPress Pro" plugin was creating this issue. Deactivating the plugin resolved the issue and everything went back to normal.

Note that "SEOPress" plugin is compatible with WPML, which was also active in the site, but the Pro version wasn't. So asked the client to contact plugin author since their base version is already working.

Relevant Documentation:
https://wpml.org/plugin/seopress/