This thread is resolved. Here is a description of the problem and solution.
Problem: I am trying to translate message form inside the post and Next Post / Previous Post, but String Translation doesn't shows any string to translate. Solution: Applying the workaround presented here https://wpml.org/errata/missing-_icl_strings-_icl_string_translations-data-tables/ made the strings appear.
This is the technical support forum for WPML - the multilingual WordPress plugin.
Our next available supporter will start replying to tickets in about 2.06 hours from now. Thank you for your understanding.