Why was translating the bible considered heretical in the middle ages?

It wasn't. For the most part translation of bibles into vernacular - whether French, German, Dutch, English, etc - was carried on without comment from ecclesiastics during the middle ages. When it did become an issue we find that it is results from accusations of heresy (or heretical interpretation contained therein), and not production as a heresy in and of itself. So, prohibitions on vernacular bibles in Toulouse and other places in southern France during the 13th century were related to stamping out heresy there; likewise, the German Emperor of the 14th century Charles IV in his famous outlawing of vernacular literature was aimed at heretical sermonizing and pamphletizing; and again, edicts against literature of the Beguines in the Lowlands and the Lollards in England in the late middle ages had to do with the content of preaching and not being in vernacular per se. In fact in 1199 Pope Innocent III instructed (although it was not always followed) the following to priests and bishops when there is suspicion of 'troubling behaviour' due to heretical beliefs incorporated in vernacular writings:

Inquire and find out the truth: who was the translator of that translation, what was the intention of the translator, what do its users believe, why do they teach, and do they uphold the apostolic see and the catholic faith?

The issue of vernacular bibles and Martin Luther had nothing to do with being translated into German or any other language. It had to do with Luther (and other reformers') suggestion that the truth of the bible had been obscured by the Roman Church through a thousand years of institutional, corruptive worldly powers, and the concomitant ideological interpretation of the bible through allegory. For that reason Luther taught and preached that ecclesiastics must go back to the original writings in Greek and Hebrew and re-translate them so that they can be experienced by Christians 'directly', basically bypassing on the Catholic constructions upon Jerome's Vulgate Latin Bible. Of course this was in the context of his other complaints about the trappings and rituals of the Roman Church which had 'distorted' the meaning of Christ and the apostolic works.

For its part the Catholic ideology was a medieval edifice constructed upon Jerome's Vulgate in Latin: the bible interlocked with canon law, canon law interlocked with decretals, letters and bulls, and the bible and canon law - upon which the church depended - were glossed in Latin for explanation of meaning. The papacy of course recognized that preaching would be in the vernacular, but the foundation, including carefully prescribed rituals, were conducted according to the Word of God and that Word was Latin. Ecclesiastics, whether university students, masters, theologians, of course knew that the sources were in Greek and Hebrew, but they viewed any retranslation efforts as being a matter of reconciling 'errors' back to Jerome's Vulgate Latin, even if that meant allegorizing said 'errors' to find reconciliation, and thus no error at all.