Thank you. I found out that it was a issue with the acf-qtranslate plugin, not with ACF, because acf-qtranslate has not been updated over a year and some lines of code have changed in the ACF plugin.
But this fork works: https://github.com/fburatti/acf-qtranslate
Anyway the original question is more precisely: How to build a multilingual gallery field.
And I think the answer lies in analysing the acf-qtranslate plugin and reading about “Creating a new field type” https://www.advancedcustomfields.com/resources/creating-a-new-field-type/
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