I tried to find examples and see whether I could see a pattern:
Athenae -- Atheniensis
Carthago -- Carthaginensis
Herculaneum -- Herculanensis
but also:
Neapolis -- Neapolitanus
Roma -- Romanus
Syracus -- Syracusanus
So... Atlantidensis? Atlantidianus? Atlantidines? Something else?
What would you suggest?
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[1] www.wl.widelands.org
[2] https://translations.launchpad.net/widelands