Texts are not translated

I'm developing an editor widget and having trouble with the translation. What I've done so far is:

• Created a directory called "language"

• Created a file called "de.json"

• Add the translations to "de.json":

  libraryStrings: {
    "loading": "Laden..."
  }

• Add the following lines to the end of my script:

H5PEditor.language['H5PEditor.MyWidget'] = {
  libraryStrings: {
    "loading": "Loading..."
  }
};

In the source I use "H5PEditor.t('H5PEditor.MyWidget', 'loading') to translate the text. Finally, I use "h5p pack -r MyWidget" to create the h5p file. But unfortunately, it's not translating to German. I always see the English word "Loading...". I ensured that the "de.json" is part of the h5p file created. Any suggestions why it's not translating? I'm already award of https://h5p.org/documentation/for-developers/translate-h5p-libraries but I think I've done everything correctly...