This thread is resolved. Here is a description of the problem and solution.
Problem:
How can our translation manager get to Taxonomy translation tab to translate all the categories? He doesn't have the rights to get there. And doing it in the Categories in the WP administration is not good...
Solution:
One possibility would be to make the user role of this translation manager as an 'Editor'. Since WPML 4.6.7 the editor role now has the capability to edit all translation (including taxonomies) which was possible only with admin roles before this WPML version.
Another option, depending on how you're setting the user role for the translation manager, would be to use this code: https://wpml.org/forums/topic/you-dont-have-the-rights-to-translate-from-nl-to-es/#post-14738667
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