Now you can define the "must fill fields" by language. Add the Language ID in front, before the field.

Example: (required in all languages)

oxuser__oxstreet

Example: (required only in language 1 (English). This can exists more often for different languages)

1|oxuser__oxstreet

Example:

1. Copy files into following directory

    source/modules/rs/mustfillfields
   

2. Add to composer.json at shop root

    "autoload": {
        "psr-4": {
            "rs\\mustfillfields\\": "./source/modules/rs/mustfillfields"
        }
    },
   

3. Refresh autoloader files with composer in the oxid root directory.

    composer dump-autoload
   

4. Enable module in the oxid admin area, Extensions => Modules

5. Make your changes in the oxid admin