I demand that Udo Richter may or may not have written...
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How does this translate into gettext? Does each plugin use its own textdomain(), and if, how does the domain switching happen?
It should; see dgettext(3).
Will plugins based on the old i18n.c concept continue to work, or do they have to migrate to gettext?
I'd give them some time (and output warnings).
Will a converted plugin work under a non-gettext VDR version too?
I see no reason why it shouldn't.
For the existing plugins, we imho need a smooth upgrade path, preferably some scripting-fu that turns i18n.c into .po files. [...]
I don't think so. Translatable strings should be marked up for extraction in the standard way: let the existing gettext tools do the work.