When and why did Germanic languages (and more in general, languages outside Italy) started to use Andrea as a female name? To my rough understanding of Greek, this is a male name, which comes from the Greek "andrós," that indicates "opposed to the woman."
My strong suspicion is that Andrew came into use outside the Greek language almost entirely as a product of Christianization (as this is the name of one of the twelve disciples of Jesus) which would have penetrated Southern Germany around the 3rd or 4th century of the common era, and would have reached Northern Germany several centuries later. The translation of the Bible into a Germanic language was into Gothic in the 4th century CE, was made from the Greek version, and this apparently was a precedent for translation conventions in all subsequent editions, most of which were not made until the late Middle Ages. Until the Protestant Reformation and Luther's famous German bible translation, however, most Germanic Christians would have used a Latin mass and bible accessible verbatim only to the clergy.
Andrea was probably adopted as a female form of Andrew at some time after the original Greek meaning was lost, which probably would have been almost immediately in Germanic languages, as the Biblical context would not have conveyed the underlying Greek meaning.
The relevant Wikipedia article provides the Biblical context that is the origin of the name but doesn't clarify the answer to your exact question.
Andrea is a male name in Italian and Albanian, and may have become a female name in Germanic languages because the ending sounds feminine in those languages. (Similarly, while the Italian job title "barista" sounds feminine in English, it is actually unisex in Italian.)
Regarding my earlier answer about the Proto-Basque origin. My update to make more clear was rejected by formal reasons, I have to add this as a short new answer with new sources.
The question was: "When and why did Germanic languages (and more in general, languages outside Italy) started to use Andrea as a female name?" And we have to add an English language perspective for this Stack Exchange?
Imaginatively, it was with the background of the Proto-Basque and the further development of culture like maybe not so much by the Phoenicians, as by, the Celtic expansion in Western Europe, like through today nations of Spain, France and British Islands, Germany which cross breading culture and language with the older Basque languages as much as the later medieval events? https://en.wikipedia.org/wiki/Celts#/media/File:Celtic_expansion_in_Europe.png
Further, what happened in the development leading to the use of Andrea as a female name outside Italy continue to use Andrea as a female name might be found here in the following quotes about female names during the Roman Empire and the Christian Medical era with the use of Greek and Latin of the church that follow the so called Pagan era?
“In the earliest period, both men and women used praenomina. However, with the adoption of hereditary surnames, the praenomen lost much of its original importance. Women's praenomina gradually fell into disuse, and by the first century the majority of Roman women either did not have or did not use praenomina. ..In Late Antiquity, women were frequently named for their mothers or other female relatives, who in turn were often named for female (or sometimes male) Christian saints. …Empresses bearing pagan names—e.g. Aelia Eudocia, formerly Athenaïs—were renamed to have more Christian names, sometimes for an earlier empress. A few empresses such as Theodora, wife of Justinian, were also allegedly renamed. Late Byzantine empresses bore names derived via Latin from Greek.”
“A great variety of names are attested from the medieval period, falling into the rough categories of Scandinavian (Old Norse), Anglo-Saxon (Old English), continental (Frankish, Old High German and Low German), and East Germanic (see Gothic names3) forms. By the High Middle Ages, many of these names had undergone numerous sound changes and/or were abbreviated, so that their etymology is not always clear.”
“After the Norman conquest most Old English and Norse forenames disappeared. Within the Christian faith, the forename was the name given at the christening (baptism) ceremony and it became known as the 'Christian name'.
The Normans also introduced the idea of female names formed from male names - Patrick/Patricia, Paul/Paula. […] During the twelfth and thirteenth centuries, New Testament biblical names came into fashion -Andrew,[…]”
Andrea in English Etymology 1
Latinate feminine form of Andreas and Andrew.
Recorded since the Middle Ages, but first popular in the latter half of the 20th century.
Classical Latin typically made a grammatical feminine gender in “a” (silva – forest, aqua – water) and this was reflected in feminine names originating in that period, like Emilia. …In the Germanic languages the female names have been Latinized by adding “e” and “a”: Brunhild, Kriemhild and Hroswith became Brunhilde, Kriemhilde and Hroswitha.
Dear friends of language,
I am not sure, I am only a true hobbyist not a true linguist, I might be completely wrong, but I think the word Andrea is a female name derived from one of the oldest languages in Europe, older than the Greek language, namely, Proto-Basque, where its meaning is a lady/ young/ woman or wife, probably spreading by the Phoenicians from the Iberia island. Only in the very late part of antiquity the influence of the Greek language, for example the island Cyprus becomes Greek only something like 400 BC.
Greek language, an Indo European language, spread the new meaning Andros "man" to the Eastern part of the Mediterranean sea with the new Rome of Italy namely the byzantine Roman empire where Greek became the language of the administrators.
Hence, Italy and the countries of the Greek Orthodox churches in Eastern Europe, Andrea is in these locations a male name. Culture is a coy science, so I guess it took some twists during the middle age during events like the English invasion of France and Aquitaine (Basque speaking)? I might be wrong of old theses ideas. Probably, as well some kind of mixed blessings did run the development?
Imaginatively, maybe not by the Phoenicians, so much as by, the Celtic expansion in Western Europe, like through today nations of Spain, France and British Islands, Germany which cross breading culture and language with the older Basque languages as much as the later medieval events?
Yellow: Core Hallstatt territory, by the sixth century BC
Pale green: Maximal Celtic expansion by 275 BC
Dark green: Areas where Celtic languages remain widely spoken today
Basque-Aquitanian ancient form "andere(a)", present-day "andere(a)" and "andre(a)
Often compared to Proto-Celtic *anderā (“young woman”), which is of an unknown origin.
- Basque is geographically surrounded by Romance languages but is a language isolate unrelated to them, and indeed, to any other language in the world.
- It is the last remaining descendant of one of the pre-Indo-European languages of Western Europe, the others being extinct outright.