One can have a multi-party system yet not have multiculturalism. Political parties and cultures are different kinds of social groupings. Monism and pluralism are the largest philosophical differentiators; one universal substance with a plurality of forms. Metaphysics has a different lexicon than social philosophy though some of the words are the same; the meanings differ.
Political monism could be taken to be monarchy; pluralism might be regarded as democracy and multiculturalism as confusion or chaos, though not necessarily. Political pluralism differs from authoritarian systems; i.e. dictatorships, communism and other one-party or no-party systems. Plainly China could have one culture and many parties (pluralism) if it was an open society. A culture would seemingly be dominant geographically or on the road to extinction. Cultures may be many things with some subcultures of trivial stature and others upsurging into being a dominant culture. Political cultures and political economy exist within the full spectrum of political and government forms of course. Pluralism could even mean cultural pluralism =multiculturalism. Multiculturalism could refer to social groupings besides those of racial or national composition as primary characteristics, and just point to Goths or hip hop cultures and so forth. There is a lot of overlap in the terminology.
I read an article recently about the interaction of human culture with Neanderthalers, and why the Neanderthals became extinct. There was quite a large period of overlap where the two cultures intersected and shared genes. The article hypothesized that human illnesses brought from Africa to Europe slowly killed off the Neanderthals that had no immunity to the diseases humans brought, while the Neanderthals had fewer lethal diseases to decimate humans. Eventually humans overcame the burden of illness/immunity problem before the Neanderthals and about 40,000 years ago I believe it was the last of the Neanderthals faded away.
For a time the hybrid human-Neanderthals may have had a pluralist social environment that was multicultural too. If the hybrids shared the same tool-kit and lifestyle plus art and religious belief then they would have been monocultural. In that case if they elected one chief for all, perhaps including volunteer pure-breed Neanderthals and humans, they could be said to be politically pluralist or monistic with little loss of meaning. I would favor pluralism since pure bred humans, pure bred Neanderthals and hybrids may have held different political values.