Y Gododdin, or The Gododdin, is believed to be the oldest extant poem in the Welsh language, dating sometime from the 7th century. (And therefore, of course the oldest extant poem in any modern European language.)

Its authorship is attributed to one Aneirin or Aneurin, since the earliest record of the poem is in Llyfr Aneirin, the Book of Aneirin, a 13th century manuscript, now held by the Cardiff Library (Cardiff MS 1). The manuscript itself contains two different but overlapping versions, which indicates the existence of separate versions of the poem before they were copied into the manuscript. (And therefore probably that the original verse had mutated somewhat over the centuries

The title Gododdin, refers to the kingdom of the Gododdin (the Votadini). According to the poem their king, Mynyddog Mwynfawr, sends out an army of 300 warriors to do battle at Catraeth against the Saxons of Deira and Bernicia. The Celts slaughter one army of Saxons, get drunk, and then get killed in turn when a second lot of Saxons turn up. The poem is therefore an elegy, or one long series of elegies for the fallen warriors of the Gododdin.

Historically speaking the battle of Catraeth is assumed to have taken place sometime around 605 AD; traditionally people have identified Catraeth as modern Catterick, but that is no more than supposition. More recently it has been suggested that Catraeth was located somewhere on the borders of modern Powys and Shropshire.

An extract from Y Gododdin first in the original Welsh;

Gwyr a aeth Ododdin, chwerthin wanar,
Disgyniaid ym myddin, trin ddiachar,
Wy lleddynt a llafnawr heb fawr drydar.
Colofn glyw, Rheithfyw rhoddi arwar.
Gwyr a aeth Gatraeth, oedd ffraeth eu llu,
Glasfedd eu hancwyn a gwenwyn fu,
Trychant trwy beirant yn catau,
A gwedi elwch tawelwch fu.
Cyd elwynt lannau i benydu,
Dadl ddiau angau i eu treiddu

then in an English transalation by AOH Jarman;

Warriors went to Gododdin, with eager laughter,
Attackers in a host, savage in battle,
They slew with blades without much noise.
Rheithfyw, pillar of battle, delighted in giving.
Warriors went to Catraeth, their host was swift,
Fresh mead was their feast and it was bitter,
Three hundred fighting under command
And after the cry of jubilation there was silence.
Though they went to churches to do penance,
The certain meeting with death came to them.

and finally in an alternative transalation by Joseph Clancy.

Men went to Gododdin, laughing warriors,
Assailants in a savage war-band
They slaughtered with swords in short order,
War-column of kind-hearted Rhaithfyw.

Men went to Catraeth, keen their war-band.
Pale mead their portion, it was poison.
Three hundred under orders to fight.
And after celebration, silence.
Though they went to churches for shriving,
True is the tale, death confronted them.

General comment - Of the two translations the latter seems the closest to the original. Note that most of the Welsh text is perfectly decipherable to anyone who speaks modern Welsh. Decipherable, because one has to take into account that its written in a particularly dense, poetic language and that it uses a lot of archaic words that you need to look up in a good dictionary. But one could still get the general sense of it.