Cornish is a Celtic language, and is part of the Brittonic group of Celtic languages (having evolved from common brittonic). It is most closely related to Breton and Welsh.
The language became extinct in the 18th century, however shortly after its extinction a revival movement started, and nowadays there are several thousand fluent speakers.
The process of revival hasn't always been smooth particularly regarding orthography with three contenders possible : Kernewek Kemmyn (common Cornish), Revived Late Cornish and unified Cornish.
A Standard Written Form was agreed on in 2008 via the Cornish Language Partnership after protracted negotiations.
Cornish (as revived) is based on Middle Cornish (medieval Cornish), and the literature written during that period (1200-1600), most importantly the Ordinalia.
Breton and Welsh have also been used as aids in the reconstruction of the language.
The numbers 1 to 10 in other brittonic languages: Breton, Welsh.
The days of the week in other brittonic languages: Breton, Welsh.
Place names in the brittonic languages as expected show striking similarities. For example, the Cornish city Falmouth Aberfala (mouth of Fal) and the Welsh city of Swansea Abertawe (mouth of the Tawe).
See Ein Tad and Hon Tad for a comparison with the Lords prayer in Welsh and Breton respectively.
There are different surviving versions of Pader Agan Arluthsome of which can be seen on http://www.bibelkernewek.com.
Agan Tas ni, eus y’n nev,
bennigys re bo dha hanow.
Re dheffo dha wlaskor,
Dha vodh re bo gwrys y’n nor kepar hag y’n nev.
Ro dhyn ni hedhyw agan bara pub dydh oll,
ha gav dhyn agan kammweyth
kepar dell evyn nyni
dhe’n re na eus ow kammwul er agan pynn ni;
ha na wra agan gorra yn temptashyon,
mes delyrv ni dhiworth drog.
Rag dhiso jy yw an wlaskor,
ha’n galloes ha’n gordhyans,
bys vykken ha bynari.
Amen.
The Cornish language board
The Cornish language society
Cornish dictionary