The translation between language code and language name (discussed at meta page linked above) is already available elsewhere on every decent wiktionary. This is about reading out the RAW heading and nothing else.

(For future reference, please use the feature request form (linked from the top of the task creation page) to create feature requests, and fill in the sections in the template. Thanks!)

Scribunto takes extra steps to avoid modules accessing any part of the wikitext outside of the current {{#invoke:}} call (in particular, to avoid one module call passing data to the other one). In light of this, I find it quite unlikely that this feature will be ever implemented…

Looks like T122934 is relevant and would help with this. Unfortunately, there's been no movement on that task recently.

Indeed T122934 adresses same problem, but this one offers a simpler and cleaner solution avoiding the need for extra code like {{#sectionproperty: declare | lang=en}}.

Particularly @TTO interesting comment: T122934#1921615

in particular, to avoid one module call passing data to the other one

I am not yet fully convinced that this is needed, still this function "read out previous section heading" can be implemented in a way not allowing intermodule communication, namely by accepting only headings consisting of raw wikitext, ie ignoring those brewed by templates and modules.

I also greatly prefer T122934 (or something similar) to this hacky workaround.