“Luther's Qualifications. Luther had a rare combination of gifts for a Bible translator: familiarity with the original languages, perfect mastery over the vernacular, faith in the revealed word of God, enthusiasm for the gospel, unction of the Holy Spirit. A good translation must be both true and free, faithful and idiomatic, so as to read like an original work. This is the case with Luther's version. Besides, he had already acquired such fame and authority that his version at once commanded universal attention.
His knowledge of Greek and Hebrew was only moderate, but sufficient to enable him to form an independent judgment. What he lacked in scholarship was supplied by his intuitive genius and the help of Melanchthon. In the German tongue he had no rival. He created, as it were, or gave shape and form to the modern High German. He combined the official language of the government with that of the common people. He listened, as he says, to the speech of the mother at home, the children in the street, the men and women in the market, the butcher and various tradesmen in their shops, and, "looked them on the mouth," in pursuit of the most intelligible terms. His genius for poetry and music enabled him to reproduce the rhythm and melody, the parallelism and symmetry, of Hebrew poetry and prose. His crowning qualification was his intuitive insight and spiritual sympathy with the contents of the Bible.
A good translation, he says, requires "a truly devout, faithful, diligent, Christian, learned, experienced, and practiced heart."”

Dr. Emser, one of the most learned opponents of the Reformation, singled out in Luther's New Testament several hundred linguistic blunders and heretical falsifications. Many of them were silently corrected in later editions. He published, by order of Duke George of Saxony, a new translation (1527) for the purpose of correcting the errors of "Luther and other heretics."
Notable examples of Luther's renderings of Hebrew and Greek words
Source: Annotationes des hochgel. und christl. doctors Hieronymi Emsers über Luthers neuw Testament, 1523. Emser charges Luther with a thousand grammatical and fourteen hundred heretical errors. He suspects (p. 14) that he had before him "ein sonderlich Wickleffisch oder Hussisch Exemplar." He does not say whether he means a copy of the Latin Vulgate or the older German version. He finds (p. 17) four errors in Luther's version of the Lord's Prayer: 1, that he turned Vater unser into Unser Vater, against the German custom for a thousand years (but in his Shorter Catechism he retained the old form, and the Lutherans adhere to it to this day); 2, that he omitted der du bist; 3, that he changed the panis supersubstantialis (überselbständig Brot!) into panis quotidianus (täglich Brot); 4, that he added the doxology, which is not in the Vulgate. In our days, one of the chief objections against the English Revision is the omission of the doxology.
Source: Das gantz New Testament: So durch den Hochgelerten L. Hieronymum Emser seligen verteutscht, unter des Durchlauchten Hochgebornen Fürsten und Herren Georgen Hertzogen zu Sachsen, etc., ausgegangen ist. Leipzig, 1528. The first edition appeared before Emser's death, which occurred Nov. 8, 1527. I find in the Union Seminary four octavo copies of his N. T., dated Cöln, 1528 (355 pp.), Leipzig, 1529 (416 pp.), Freiburg-i.-B. 1535 (406 pp.), Cöln, 1568 (879 pp.), and a copy of a fol. ed., Cologne, 1529 (227 pp.), all with illustrations and marginal notes against Luther. On the concluding page, it is stated that 607 errors of Luther's are noted and corrected. The Cologne ed. of 1529 indicates, on the titlepage, that Luther arbitrarily changed the text according to the Hussite copy. Most editions contain a Preface of Duke George of Saxony, in which he charges Luther with rebellion against all ecclesiastical and secular authority, and identifies him with the beast of the Apocalypse, Rev. 13.

How Luther's theology may have influenced his translating

Ulfilas's Gothic translation

Roman Catholic rival German versions of the Bible
Context: A Critical Estimate of Luther's Version. Luther's version of the Bible is a wonderful monument of genius, learning, and piety, and may be regarded in a secondary sense as inspired. It was, from beginning to end, a labor of love and enthusiasm. While publishers and printers made fortunes, Luther never received or asked a copper for this greatest work of his life.

Source: The best critic of a translation is its second translation, Center for the Great Islamic Encyclopedia, 2013 https://www.cgie.org.ir/fa/news/3001

German versions of the Bible that preceded the Luther Bible
Source: Luther's use of the older German version was formerly ignored or denied, but has been proved by Professor Krafft of Bonn (1883).

Notable examples of Luther's renderings of Hebrew and Greek words

Source: 300 Tang Poems: A New Translation (1987), p. xxii

Sto de tak kráto naſim med Mürom i Rábom prebívajoucſim ſzlovenom tè ſz. Bo'ze knige na ſzvoj jezik, po ſterom ſzamom li vu ſzvoji Prorokov i Apoſtolov píſzmaj gucsécsega Bogà razmijo, obracsati? geto je nyim zapovidáva Goſzpodin Boug ſteti; da je moudre vcſiníjo na zvelicſanye po vöri vu Jezuſi Kriſztuſi; tou pa ni ſzTruberovòga, ni Dalmatinovoga, ni Frenczelovoga, niti znikakſega drügoga obracsanya (verſio) csakati ne morejo. Ár tej naſ Vogrſzki ſzlovenov jezik od vſzej drügi doſzta tühoga i ſzebi laſztvinoga mà. Kakti i vu naprek zracsúnani ſze veliki rázlocsek nahája. Zâto je potrejbno bilou tákſemi csloveki naprej ſztoupiti: kíbi vetom delao Bougi na díko ‘a’ ſzvojemi národi pa na zvelicsanye. Liki je i Goſzpodin Boug na tou nadigno Stevan Küzmicsa Surdánſzkoga Farara: kí je zGrcskoga pouleg premoucſi i pomáganya Dühà ſzvétoga zvelikom gyedrnoſztjom na ete, kákſega ſtés i csüjes, jezik czejli Nouvi Zákon obrnyeni i ſztroskom vnougi vörni düsícz vö zoſtámpani i tebi rávno tak za toga zroka, za ſteroga volo ti je 'z pred temtoga od nyega ſzprávleni Vöre Krſztsánſzke Krátki Návuk.Foreword of the Nouvi Zákon