圣诞节快乐 (pinyin romanization "shengdanjie kuaile") = Merry Christmas
新年快乐 or 恭喜发财 (pinyin romanization for first phrase "xinnian kuaile"; for the second phrase "gongxi facai") = Happy New Year
p.s. All pronounciation with the Mandarin dialect
OK,I could not say that "圣诞节快乐"is wrong,but it sounds "non-orthodox".During the period of Nativity,we greet with"合利斯托斯降生哉“（hristos jiang sheng zai——Χριστὸς γεννᾶται),and the answer is:"眾宜予榮贊” （zhong yi yu rong zan——δοξάσατε).
By the way,we do not call Nativity as 圣诞（“圣诞”is the term traditionally indicate the birth of a sage like Confucius or Mencius;or the birth of one emperor. )We call Nativity as "主降生（the Gennesis of the Lord)“or "救世者降生大瞻礼(the great celebration of the Gennesis of the Saviour)”.
Of course this is the way we EO do, I do not know how about the coptic chinese. (Since they are under the diocese of Australia,I suppose that they use only coptic and english in their religious life).