I got bits and pieces of translations from the net and dictionary, and this is what I came up with (my translation of verse#3 might be a bit off, though):
REFRAIN: Parce, Domine, parce populo tuo:
ne in aeternum irascaris nobis.
Spare, Lord, spare your people:
Be not angry with us forever
1. Flectamus iram vindicem,
Ploremus ante Judicem;
Clamemus ore supplici,
Dicamus omnes cernui:
Let us appease His wrath,
Beg for mercy from our Judge;
Cry to Him in supplication,
Let us all prostrate and say:
2. Nostris malis offendimus
Tuam Deus clementiam
Effunde nobis desuper
Remissor indulgentiam.
Refrain
By our sins we have offended
against your mercy, O God
Pour forth from above
O pardoning One, your forgiveness
3. Dans tempus acceptabile,
Da lacrimarum rivulis
Lavare cordis victimam,
Quam laeta adurat caritas.
Having given us this acceptable time,
grant that in the water of our tears
we may purify our heart and that it may become
a joyful sacrifice offered out of love.
4. Audi, benigne Conditor,
Nostras preces cuм fletibus
In hoc sacro jejunio,
Fusas quadragenario.
Refrain
O Merciful Creator, hear
our prayers with our weeping
in this holy time of
forty day fasting.