This thread is resolved. Here is a description of the problem and solution.
Problem:
The client is concerned about whether it is safe to update WordPress language files when using WPML and if it will affect the translations made via WPML.
Solution:
We assure you that it is safe to update the WordPress language files. When you run these updates, WordPress will automatically place the new translation files in the correct folder. Moreover, if you have made any translations related to WordPress in WPML's String Translation, WPML will create a new .mo file that includes those translations.
If this solution does not seem relevant to your issue, please do not hesitate to open a new support ticket with us.
This is the technical support forum for WPML - the multilingual WordPress plugin.
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This topic is split from https://wpml.org/forums/topic/translated-text-not-displayed-in-the-backend/