Why it happens
The error 404 on the th/<page slug> pages occurs because your custom rewrite rules overwrite the default Page rules which would otherwise load the correct Page (having the slug <page slug>).
How to fix the issue
If your permalink structure is /%postname%, i.e. example.com/<post slug>, then a simple fix to the error 404 is by using the post_rewrite_rules hook instead of rewrite_rules_array.
So all you need to do is:
// Change this:
add_filter( 'rewrite_rules_array','customLangaugeSlugRules');
// to this one:
add_filter( 'post_rewrite_rules','customLangaugeSlugRules');
Then flush the rewrite rules — but as the WP_Rewrite::flush_rules() documentation says: (*slightly reformatted)
Because this function can be extremely costly in terms of
performance, it should be used as sparingly as possible – such as
during activation or deactivation of plugins or themes. Every attempt
should be made to avoid using it in hooks that execute on each page
load, such as
init.
So keep that in mind, and you could actually flush the rules manually by simply going to the Permalink Settings admin page (wp-admin → Settings → Permalink).