There are also external metadata stored in Wikidata for some wiki projects. More are needed (e.g. for different typing sort keys).

Any way I expect to see soon a reliable way to detect the page language including for translated pages; but more importantly for sources of translations without having to assume they are in English, or create thme in another language and creating a pseudo-translation to the original language by copying keys, then modifying the English source again but keeping the original text.

At least, when we mark a new page for translation, we should immediately have an option asking in which language is the source; if it's not specifid by the new experimental Special:PageLanguage page (which is not necessarily needed).

And once a source page has been marked for translation, the Translate tool should have a simple API to query its language or the language used in the generated translations, And ideally, we should be able to swithc from one source language to another (for example some projects start in English, but are later managed in German or Chinese, or a local Chapter initially creates documents in its own local language such as French, Hindi or Spanish, and will not use English as the reference (this is important for pages reporting local projects mostly done in other languages, outside countries or regions with a majority of native English-speakers, i.e: most countries of the world, including Europe (and even North America where French and Spanish are very present too ; Spanish and Chinese are also growing fast in US, and here there are aslo local communities that would like to promote their own local projects in their native non-English tongue : do you remember that US does not have any "official" language ?).