This thread is resolved. Here is a description of the problem and solution.
Problem: I'd like to set up my Field Groups in English, instead of the default language of German for ease of use during development. Is there any issues in doing this since it is not the default language?
Solution: It should be not any problem to use the site language as German and start working on ACF in English. If you set a ACF field to copy, it will take the language value of currently used language for ACF. Afterwards you will still be able to translate the backend, in order for other German users to use the plugin. This translation will not take any affect that on previous entered custom field values.
Relevant Documentation: https://wpml.org/documentation/related-projects/translate-sites-built-with-acf/
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