This thread is resolved. Here is a description of the problem and solution.
My site is hosted on SiteGround and I cannot move from PHP 7.0 to 7.1 due to WPML incompatibility according to the compatibility test tool from Siteground.
The Siteground script is a "simple" one. It does not check for actual LOGIC that WILL be executed. In this case, WPML has backward compatibility with pre PHP 7.1 versions and for PHP 7.1 versions and above. The script, however, does not know which one of the functions will be used, and because it finds them both - it outputs the error message because it detects the old one - which will be executed only if you have lower PHP version.
TL;DR; it is safe to use PHP 7.1
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