Synonyms for Underdeveloped (same or very similar meaning)
WordNet sense 1 (not yet mature):
immatureWordNet sense 2 (relating to societies in which capital needed to industrialize is in short supply):
developingWordNet sense 3 (not having highly developed manufacturing enterprises):
nonindustrialWordNet sense 4 (lacking contrast or shading between tones):
flatFrom the ODE community, based on WordNetadd/edit