Apparently there is no way to manually set the language of the original page marked for translation and, if I understand correctly, it's done on per-wiki basis. Which makes no sense for multi-language projects, like meta or outreach wiki.
Consider this example: I have a lengthy document in Polish I would like to share with the community (here it is at Outreach). I also translated parts of it to English here, I would love to translate it to additional languages. The easiest way would be to mark the Polish original with <translate> tags, finish the translation to English (copy-pasting paragraphs one by one), and then translate either the English or Polish version to additional languages. Except MediaWiki automagically assumes the original text is in English.
There should be a way to manually set the language of the original document. When translating, there should also be a way to chose another language as a source (say, in our case someone might want to translate from English rather than from Polish).