This thread is resolved. Here is a description of the problem and solution.
Problem: Unable to translate image alt tags or alt tag is missing in translation editor
Solution: You may try this alternative solution:
If you want to translate Alt Texts and Titles: - Go to Media Library - In the top bar menu, select your language of interest : http://take.ms/xI7qw - Select an image and edit its attributes.
Actually, when you change language on the top menu, you are also changing the language for Media library, so, every change you make in your images will be reflected in your selected language.
You can find more information here: https://wpml.org/documentation/getting-started-guide/media-translation/
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