The text is:
O magnum mysterium,
et admirabile sacramentum,
ut animalia viderent Dominum natum,
iacentem in praesepio!
Beata Virgo, cujus viscera
Dominum Iesum Christum.
One more-or-less standard English translation is:
O great mystery,
and wonderful sacrament,
that animals should see the new-born Lord,
lying in a manger!
Blessed is the Virgin whose womb
was worthy to bear
the Lord, Jesus Christ.
Every time I’ve sung it, I’ve been struck by how many of the Latin words have engendered English words. English is officially classified as a Germanic language, but many of its advanced words are derived from Latin. In fact, two of the words are Greek and two are Hebrew through Greek. Some words came into English via French rather than directly from Latin.
Here are the Latin words again, with some of the English words they have engendered (mainly from Wikipedia). There are definitely more, given the English processes of prefixing and suffixing, and some very rare words:
O – O, oh
magnum (magnus/magna/magnum, great, large) – magnanimity, magnanimous, magnate, magnificent, magnify, magnitude, magnum
mysterium (μυστήριον, mystḗrion, mystery, sacrament) – mystery, mysterious, mystify, mystic(al), mysticism, mystique
et – none, but the ampersand & is a stylised abbreviation of this word.
admirabile (ad + mirus, wonder, amazement) – admire, admirable, admiration, marvel, marvelous, miracle, miraculous, mirage, Miranda, mirror
sacramentum (sacer/sacra/sacrum, sacred, holy) – consecrate, desecrate, sacrament, sacred, sacrosanct
ut – none, but ‘that’ is a direct equivalent
animalia (which is derived from anima breath, life, spirit) – animal/animalia, anima/animus, animate, animation, animator, animatronic, animadvert, animism, animist
viderent (videre/video/visus, to see) – advise, clairvoyant, envisage, envision, envy, evidence, improvise, interview, invidious, preview, provide, proviso, prudent, purvey, purview, review, revise, supervise, survey, videlicet, video, view, vis-à-vis, visa, visage, visible, vision, visit, visor, vista, visual, voilà, voyeur
Dominum (dominus, master, from domus, house) – belladonna, condominium, dame, damsel, danger, demesne, demoiselle, domain, dominant, domineer, dominion, domino, duenna, dungeon, madam, mademoiselle, madonna, predominant
natum (nascere/nāsci/natus, to be born) – adnate, agnatic, cognate, innate, nada, naïve, nascent, natal, nation, native, nature, née, pregnant, renaissance
iacentem (iacere/iaceo/iacitus, to be thrown) – adjacent, ease/easy, joist
in – in
praesepio (prae– + sepio). Latin prae– gives English pre- and all the words using that prefix (there’s one!). Saepire/saepio/saeptum, to surround, enclose, fence in; envelop, wrap. The only related English word is transept.
Beata (beare/beo/beatum, to bless) – beatify
Virgo (virgō/virginis, maiden) – virgin
cuius – English whose is distantly related via Proto-Indo-European *kʷis. Compare Latin quis, quem, cuius and English who, whom, whose.
viscera (viscus/visceris, internal organ) – eviscerate, visceral.
meruerunt (merērī/mereo/meritus, to earn) – merit
portare (portare/porto/portatum, to carry) – comportment, deport, export, import, port, portable, portage, portal, porter, portfolio, purport, rapport, report, support, transport
Dominum – see above
Iesum (Hebrew ישוע Yehoshua, ישוע Yeshua, Greek Ἰησοῦς Iesous, Latin Iesus Jesus, Jesuit
Christum (Greek Χριστός Christos, Latin Christus) – Christ, Christian, Christianity, Christmas
Alleluia! (Hebrew הללו יה, Hallelujah, Greek ἀλληλούϊα)
These last three have been taken almost intact into other languages. I’m not aware of any language which translates them rather than transliterating them. For example, in Korean, they are 예수 크리스토스 (ye-su keu-ri-seu-to) and 할렐루야 (hal-le-lu-ya).
Some of the derivations aren’t immediately obvious. I checked a few, and they are there.