Will be in Taiwan in Dec, will there be any Xmas or new year celebration in Taiwan/Taipei?

Yes, there will be celebrations for both Christmas and New Year's in Taipei. Hotels will put out special menus for Christmas, while most hypermarts will have special sales. As for New Years just head down to Taipei 101- there's always an awesome display of fireworks. The Taipei City government as well as other major county/city government should also have concerts on new year's eve. I suggest you ask around about the different lineups for the various concerts.