The 25th of December is the great birthday of Jesus that is celebrated by millions of people all over the world. Though no one knows the real birthday of Jesus, as the date is not given in the Bible. Then what makes us do so? Why the 25th of December? According to the Christmas Customs, the birth of Jesus probably didn't happen in the year 1AD but slightly earlier, in about 5, 6 or 7BC.

In the 5th century Christmas was first celebrated on the 25th December that was in the time of the Roman Emperor Constantine. He first bought the Roman pagan 'sun-day' (the first day of the week) and the 'sabbath' (the Christians holy day) together to what we now call Sunday. As it is mentioned in Christmas Customs, this date was probably chosen because the Winter Solstice and the ancient pagan Roman midwinter festivals called 'Saturnalia' and 'Dies Natalis Solis Invicti' took place in December.

Anyway we do celebrate Christmas on the 25th of December. People of different nationalities congratulate each other. And it is always pleasant to hear the phrase: Marry Christmas in your own language. We do congratulate our friends in their languages in order to express our respect.
And if you have a foreign friend whom you’d wish to say Marry Christmas in you can choose from these languages:

Afrikaans - Geseende Kerfees en 'n gelukkige nuwe jaar
Albanian - Gézuar Krishlindjet Vitin e Ri!
Amharic - Melkam Yelidet Beaal
Arabic - I'D Miilad Said ous Sana Saida
Aramaic - Edo bri'cho o rish d'shato brich'to!
Armenian - Shenoraavor Nor Dari yev Pari Gaghand
Aromunian - Crãciunu hãriosu shi unu anu nãu, bunu!
Azeri - Tezze Iliniz Yahsi Olsun
Basque - Zorionak eta Urte Berri On!
Belorussian - Winshuyu sa Svyatkami i z Novym godam!
Bengali - Shuvo Baro Din - Shuvo Nabo Barsho
Bicolano - Maugmang Capascuhan asin Masaganang Ba-gong Taon!
Bohemian - Vesele Vanoce
Brazil (Portuguese) - "Feliz Natal" or "Boas Festas
Breton - Nedeleg laouen na bloav ezh mat
Bulgarian - Vasel Koleda; Tchestita nova godina!
Burmese - Pyaw Shwin Thaw Christmas Par!
Catalan - Bon nadal i feliç any nou!
Cantonese - Seng Dan Fai Lok, Sang Nian Fai Lok
Cebuano - Malipayong Pasko ug Bulahang Bag-ong Tuig!
Choctaw - Yukpa, Nitak Hollo Chito
Cornish - Nadelik looan na looan blethen noweth
Corsican - Bon Natale e Bon capu d' annu
Cree - Mitho Makosi Kesikansi
Creek - Afvcke Nettvcakorakko
Croatian - Sretan Bozic
Czech - Prejeme Vam Vesele Vanoce a stastny Novy Rok
Danish - Glædelig Jul og godt nytår
Duri - Christmas-e- Shoma Mobarak
Dutch - Vrolijk Kerstfeest en een Gelukkig Nieuwjaar!
Egyptian - Colo sana wintom tiebeen
English - Merry Christmas & Happy New Year
Eskimo - Jutdlime pivdluarit ukiortame pivdluaritlo!
Esperanto - Gajan Kristnaskon
Estonian - Rõõmsaid Jõulupühi Head uut aastat ( Merry Christmas & Happy New Year )
Euskera - Zorionak eta Urte Berri On
Faeroese - Gledhilig jól og eydnurikt nýggjár!
Farsi - Cristmas-e-shoma mobarak bashad
Finnish - Hyvää Joulua or Hauskaa Joulua
Flemish - Zalig Kerstfeest en Gelukkig nieuw jaar
French - Joyeux Noël et Bonne Année!
Frisian - Noflike Krystdagen en in protte Lok en Seine yn it Nije Jier!
Friulian - Bon Nadâl e Bon An Gnûf
Gaelic - Nollaig chridheil agus Bliadhna mhath ur!
Galician - Bon Nadal e Bo Ani Novo
Georgian - ბედნიერ შობა-ახალ წელს!
German - Froehliche Weihnachten und ein glückliches Neues Jahr!
Greek - Kala Christougenna Kieftihismenos O Kenourios Chronos
Greenlandic - Juullimi Ukiortaassamilu Pilluarit
Hausa - Barka da Kirsimatikuma Barka da Sabuwar Shekara!
Hawaiian - Mele Kalikimaka & Hauoli Makahiki Hou
Hebrew - Mo'adim Lesimkha. Shana Tova
Hindi - Shub Naya Baras
Hungarian - Kellemes Karácsonyi ünnepeket és Boldog Új Évet!"
Iban -Selamat Ari Krismas enggau Taun Baru
Icelandic - Gleðileg Jól og Farsaelt Komandi ár!
Ilocano - Naimbag a Pascua ken Naragsac nga Baro nga Tawen!
Indonesian - Selamat Hari Natal
Iraqi - Idah Saidan Wa Sanah Jadidah
Irish -Nollaig Shona Dhuit
Italian - Buon Natale e Felice Anno Nuovo
Japanese - Shinnen omedeto.
Kurisumasu Omedeto Jèrriais - Bouan Noué et Bouanne Année
Kadazan -Kotobian Tadau Do Krimas om Toun Vagu
Karelian - Rastawanke Sinun, Uvven Vuvenke Sinun
Klingon - yItIv [Christmas] je Quch chu' DIS
Korean - Sung Tan Chuk Ha
Krio - Appi Krismes en Appi Niu Yaa
Kurdish - Seva piroz sahibe u sersala te piroz be
Ladin - Bon Nadel y Bon Ann Nuef
Lappic - Buorit Juovllat ja Buorre Oddajahki
Latin - Natale hilare et Annum Nuovo!
Latvian - Prieci'gus Ziemsve'tkus un Laimi'gu Jauno Gadu!
Lausitzian - Wjesole hody a strowe nowe leto
Lithuanian - Linksmu Kaledu ir laimingu Nauju metu
Livian - Riiemlizi Talspividi ja pagin vonno udaigastos
Low Saxon - Heughliche Winachten un 'n moi Nijaar
Luxemburgish - Schéi Krëschtdeeg an e Schéint Néi Joer
Macedonian -Streken Bozhik
Malay - Selamat Hari Natal dan Tahun Baru
Malayalam - Puthuvalsara Aashamsakal
Maltese - Nixtieklek Milied tajjeb u is-sena t-tabja!
Mandarin - Sheng Dan Kuai Le, Xing Nian Kuai Le
Manx - Nollick ghennal as blein vie noa
Maori - Meri Kirihimete
Marathi - Shubh Naveen Varsh ( Happy New Year )
Mongolian - Zul saryn bolon shine ony mend devshuulye
Monogasque - Festusu Natale e Bona ana noeva
Norwegian - God Jul og Godt Nyttår
Occitan - Polit nadal e bona annada
Oriya - Sukhamaya christmass ebang khusibhara naba barsa
Pampamgo - Malugud Pascu at saca Masayang Bayung Banua!
Papiamento - Bon Pasco y un Feliz Aña Nobo
Papua New Guinea - Bikpela hamamas blong dispela Krismas na Nupela yia i go long yu
Pashto - De Christmas akhtar de bakhtawar au newai kal de mubarak sha.
Pennsylvania German - En frehlicher Grischtdaag unen hallich Nei Yaahr!
Persian - Crismas mobarak
Polish - Wesolych Swiat i Szczesliwego Nowego Roku.
Portuguese - Boas Festas e um feliz Ano Novo
Punjabi - Nave sal di mubaraka
Pushto - Christmas Aao Ne-way Kaal Mo Mobarak Sha
Raeto-Ramance - Bella Festas da zNadal ed in Ventiravel Onn Nov
Rapa-Nui - Mata-Ki-Te-Rangi. Te-Pito-O-Te-Henua
Rhetian - Bellas festas da nadal e bun onn
Romanche - Legreivlas fiastas da Nadal e bien niev onn!
Romani (GYPSY) - Bachtalo krecunu Thaj Bachtalo Nevo Bers
Romanian - Craciun fericit si un An Nou fericit!
Russian - Pozdrevlyayu s prazdnikom Rozhdestva i s Novim Godom
Sambal - Maligayang Pasko at Masayang Ba-yon Taon!
Sami - Buorrit Juovllat
Samoan - La Maunia Le Kilisimasi Ma Le Tausaga Fou
Sardian - Felize Nadale e Bonu Cabuannu
Sardinian - Bonu nadale e prosperu annu nou
Scots Gaelic - Nollaig chridheil huibh
Serbian - Hristos se rodi
Serb-Croatian - Sretam Bozic. Vesela Nova Godina
Sicilian - Bon Natali e Prosperu Annu Novu !
Singhalese - Subha nath thalak Vewa. Subha Aluth Awrudhak Vewa
Sorbian - Wjesole hody a strowe Nowe leto.
Somali - ciid wanaagsan iyo sanad cusub oo fiican.
Slovakian - Vesele Vianoce a stastny novy rok
Slovak - Veselé Vianoce a šťastný nový rok
Slovene - Vesele bozicne praznike in srecno novo leto
Sorbian - Wjesole hody a strowe Nowe leto
Spanish - Feliz Navidad y Próspero Año Nuevo
Sudanese - Wilujeng Natal Sareng Warsa Enggal
Surigaonon - Malipayon na pasko sanan bag-on tuig!
Swahili - ºKrismas Njema Na Heri Za Mwaka Mpyaº
Swedish - God Jul och Gott Nytt A
Tagalog - Maligayang Pasko at Manigong Bagong Taon
Tamil - Nathar Puthu Varuda Valthukkal
Thai - Suksan Wan Christmas lae Sawadee Pee Mai
Tok Pisin - Meri Krismas & Hepi Nu Yia
Tongan - Kilisimasi Fiefia & Ta'u fo'ou monu ia
Trukeese - Neekirissimas annim oo iyer seefe feyiyeech!
Tswana - Keresemose o monate le masego a ngwaga o montsha
Turkish - Noeliniz Ve Yeni Yiliniz Kutlu Olsun
Ukrainian - Veseloho Vam Rizdva i Shchastlyvoho Novoho Roku!
Urdu - Naya Saal Mubarak Ho
Vepsi - Rastvoidenke i Udenke Vodenke
Vietnamese - Chuc Mung Giang Sinh - Chuc Mung Tan Nien
Waray-Waray - Maupay nga Pasko ngan Mainuswagon nga Bag-o nga Tuig!
Welsh - Nadolig LLawen a Blwyddyn Newydd Dda
Xhosa - Siniqwenelela Ikrisimesi EmnandI Nonyaka Omtsha Ozele Iintsikelelo Namathamsanqa
Yayeya - Krisema
Yiddish - Gute Vaynakhtn un a Gut Nay Yor Yoruba - E ku odun, e hu iye' dun!
Zulu - Sinifesela Ukhisimusi Omuhle Nonyaka Omusha Onempumelelo

Sources:
http://www.komar.org/christmas/merry/
http://www.whychristmas.com/customs/25th.shtml