PE 16 Sumer is icumen in
                                 c.1250; MS Harley 978, fol.11v 

Sumer is i-cumen in, lhude sing cuccu,
Growež sed and blowež med and springž že wde nu.
Sing cuccu
Awe bletež after lomb, lhouž after calve cu,
Bulluc stertež, bucke vertež, murie sing cuccu
Cuccu, cuccu,
Wel singes žu cuccu, ne swik žu naver nu.

Sing, cuccu, nu; sing, cuccu; 
One person repeats this as often as necessary, 
making a rest at the end.
Sing, cuccu; sing, cuccu, nu! 
The other one sings this, making a rest in the middle and not at the end, but immediately repeating the beginning.

Perspice Christicola
Perspice christicola, que dignacio
celicus agricola, pro vitis vicio filio
non parcens exposuit, mortis exicio
qui captivos, semivivos, a supplicio vite donat
et secum coronat, in celi solio

Notes from Lady Zenovia :
The letter 'ž' is used rather than 'th'.
The song "Sumer is icumen in" is a round, the earliest known English round, in fact, and was written for six voices.
It is recorded on only the one manuscript page. On the same page, the Latin words are written under the Middle English words, i.e. there is one tune with both Latin and English lyrics. There is no evidence to indicate whether the 2 sets of lyrics were written at the same time or at different times.

Thematically, the English version talks about spring and nature, while the Latin is religious (Easter). It's quite a nice parallel actually of secular and religious content and language.

Latin Translation: (rather awkward, but I was trying to give an idea of what particular words mean, rather than a poetic interpretation; translation is based on one from a CD)
Pay heed, christian, what an honour!
The heavenly farmer, because of a flaw on the vine
[his] son did not spare but exposed to the destruction of death;
He to the half-living captives [of hell] gives life
and crowns [them] and himself on the throne of heaven.