Agreed about bugs v improvements, I'm never sure. Another option could be an additional field for custom config, as you suggested just an input. To add languages in the Format select this was all it needed:

<customConfig>
        <!--    <config key="myKey">mySetting</config>-->
        <config key="entity_encoding">raw</config>
          <config key="style_formats">
        [
        {"title":"Headers","items":[{"title":"Header 2 (H2)","block":"h2"},{"title":"Header 3 (H3)","block":"h3"}]},
        {"title":"Languages","items":[{"title":"English","inline":"span", "attributes":{"lang":"en"}},{"title":"Chinese","inline":"span", "attributes":{"lang":"zh-Hans"}}]}
        ]
      </config>
    </customConfig>

The problem is we did not want he headings on on RTEs

I feel like there were more options on the data type editor back in v7, i'm sure you used to be able to toggle headings on and off.

I would have thought it would be a lot more work to make a package but from recent package experience 🙂