[NB: All translations from Latin and Arabic here are my own]
The Western experience of translating the Qur'an dates back nearly 900 years, beginning with one Robert of Ketton in the 12th century CE. The translation had been organized in Christian Spain as part of a wider Arabic-Latin translation project by Peter the Venerable (of the Cluny Abbey in eastern France), who wrote in a letter in Latin to Bernard of Clairvaux Abbey (also in France) regarding the endeavor vis-à-vis Islamic texts:
"But the whole impious sect, and the life of the wicked man [Muhammad], and the law, which he called the Qur'an [i.e. collection of precepts] and persuaded the most wretched of men that it had been delivered to him by the angel Gabriel from Heaven, I have nonetheless rendered from Arabic into Latin, of course with the help of interpreters skilled in both languages: Robert Retenensis [Robert of Ketton] from England, who is now the archdeacon of the church of Pamplona, along with Hermannus Dalmata, a scholar of most acute and literate character…There was in this project my intention, that I should follow that custom of the fathers, by which they have not passed over any of even the most trivial heresies (as I should thus call them) of their times in silence, without resisting them with all the strength of their faith, and showing by writings and disputations that they are to be detested and condemned.
I wanted to do this concerning this particular error of errors, concerning this torch of all heresies (I should say) of all the diabolical sects, which have arisen from the very advent of our Saviour and have continued to remain…You yourselves will recognize on reading, and thus I think you will weep (as is proper), how such a great part of the human race has been deceived by such wicked and despicable filth, and has been turned away so trivially by their founder through the nefarious sect of a most wretched man even after the grace of the Redeemer. I have especially marked out all these things for you, so that I might both make clear our studies to such a great friend, and incite that very great magnificence of yours of doctrine- which God has brought together individually for you in our days- to write against such pernicious error."
Robert of Ketton, who, like his colleague Hermannus Dalmata (originally from modern-day Slovenia), had an interest in astrology and astronomy, divided up the Qur'an into 124 chapters, rather than the standard 114 suras. There is no evidence that Hermannus played a significant role in the translation of the Qur'an.
The translation- dubbed Lex Mahumet Pseudoprophetes [Law of Muhammad the False Prophet]- is most notable for its tendency to paraphrase rather than stick strictly to the wording of the original Arabic. To take al-Fatiha as an example:
Misericorde pioque Deo, uniuersitatis creatori, iudicium cuius postrema dies
expectat, uoto supplici nos humiliemus, adorantes ipsum : suaeque manus
suffragium, semitaeque donum et dogma, quos nos ad se beneuolos, nequaquam hostes et erroneos adduxit, iugiter sentiamus.
To compare with the original, Robert of Ketton has omitted 'in the name of' (bismillah) from the opening line, and renders al-raheem ('the compassionate') as 'pious' (pius). In a similar vein, 'lord of the worlds' ('rab al-alameen') is rendered 'creator of the Universe' (creator universitatis), while maliki l-youm ad-deen ('king/lord/master of the Day of Judgment') is paraphrased 'whose judgment the last day expects' (iudicium cuius postrema dies expectat). One could go on as regards this sura.
To take another example, al-Ikhlas (sura 112, but 122 in Robert of Ketton's translation):
In nomine Dei pii et misericordis. Constanter dic illis, Deum unum esse, necessarium omnibus, et incorporeum : Qui nec genuit, nec est generatus, nec habet quenquam sibi similem.
The words 'constanter' ('with constancy') and 'illis' ('to them'- added by Robert of Ketton on his interpretation that these words would be spoken to non-Muslims) is not in the original Arabic, and Robert of Ketton has rendered direct speech by oratio obliqua (accusative and infinitive construction in Latin) as far as the first half of list of God's attributes goes. Depending on how one interprets al-samad, 'necessarium omnibus' ('a refuge for all') seems okay, yet the translator has also added 'et incorporeum' ('and He is incorporeal'), even though there is nothing of the sort in the Arabic text. I would venture that Robert of Ketton added these words to try to make a connection with the subsequent phrase: 'He has neither begotten, nor was He begotten' (which his translation renders accurately), which he presumably understands to be in answer to the Christian doctrine of God the Father and Jesus the Son as part of the Trinity.
In general, whatever authority the work commanded in subsequent centuries (and it certainly seems to have circulated more widely than another Latin translation by Mark of Toledo noted for its literalism), there is much in the spirit of paraphrase, reordering of the original material, omission and addition in Robert of Ketton's translation. Thus was it considered sorely deficient by Ludovico Marracci, an Italian priest who rendered a new authoritative Latin translation of the Qur'an first published in Padua in 1698, subsequently republished in an edition in Leipzig in 1721. Marracci had studied Arabic in Rome and eventually became confessor for Pope Innocent XI.
Like Peter the Venerable and Robert of Ketton, Marracci was not shy of his intentions to demonstrate the falsehoods of Islam, and indeed wrote a refutatio of Islam as part of the 1698 publication to accompany his translation of the Qur'an. In a similar vein, the 1721 Leipzig edition, which does not feature the refutatio, contains a lengthy preface by Christianus Reineccius- a contemporary Christian theologian- against Islam. However, it would be logically fallacious to take the anti-Islamic spirit alone to damn a translation. Instead what we find in Marracci's is a remarkable translation that sticks to the original. To take al-Fatiha as an example:
In nomine Dei, miseratoris, misericordis
Laus Deo, Domino Mundorum
Regi Diei Iudicii
Te colimus et te in auxilium imploramus
dirige nos in viam rectam
in viam illorum erga quos beneficius fuisti et non actum iracunde eos
et non in viam errantium
To give a few comparative data points: Marracci renders 'al-hamdu lillah' in his translation (laus Deo; which was omitted by Robert of Ketton) and translates 'rab al-alameen' correctly ('domino mundorum'- 'lord of the worlds') as well as 'maliki l-youm ad-deen' ('regi diei iudicii'). As for the original Arabic, 'iyyaka n'abadu wa iyyaka nast'aeenu', whereas Robert of Ketton renders the two Arabic indicative verbs by a Latin subjunctive with a present participle translating to 'let us lower ourselves to Him with suppliant vow, worshipping Him', Marracci sticks to the original construction and meaning: "We worship You and implore You for help."
Similarly, looking at Sura 112:
in nomine Dei miseratoris misericordis
Dic: est Deus unus
non genuit, et non est genitus
et non fuit illi par ullus
Leaving aside the lack of interpretative additions by Marracci and adherence to oratio recta, he also captures well the philological spirit of lam yalid wa lam yulad ("He has not begotten, nor was He begotten", with both verbs using the same w-l-d Arabic root translating to 'beget') by echoing it with the use of the same Latin root verb in translation: gignere.
Though this review of the Latin translations of the Qur'an is by no means comprehensive, it can be seen how Marracci's translation marks a great step forward in style and accuracy, regardless of the anti-Islamic sentiment. Even so, despite the problems in Robert of Ketton's work, I would not go so far as to accuse Robert of Ketton of mendacity, but rather the evidence on balance points to mistakes in an attempt to give an 'insider's perspective', so to speak, on how Muslims understand the Qur'an.