The English Bible is rife with theologically-technical terms that can be criticized, but think about it. If a language-speaking population is not to conduct all theological speech in a foreign language (Greek) then some kind of vulgar terms must be developed. In the English New Testament, some such terms are Germanic, some are Norman, some are Latin or pseudo-Latin (like your "repent"), some are transliterations (with whatever degree of precision) of Greek or Hellenized Hebrew. To my knowledge, traditionalist translators like Coverdale or the KJV committees did not invent any of these usages. The usages were developed by the English Church, over hundreds of years.
The details of how and why are unimportant -- what is important is that a term be used consistently, that it be used with understanding, and that it be used throughout a language-speaking Christian population. The criticisms that are labeled at "repent" are not legitimate by this standard. Whatever the story behind the invention of the word, the translations use it consistently and the English-speaking population has long been entirely familiar with it. (In a linguistic era such as ours, where new terms are so often concocted of acronyms, scraps of pseudo-Latin, and even numerals, I'm not sure how authentic is this sudden concern with the etymological integrity of "repent.") No, if there is any criticism to be directed at the term, it could not be at its choice in translation but perhaps at modern denominations' transmutations of the concept itself -- that is, if "repent" is harsh and unrealistic compared to μετάνοια (I'm not agreeing that it is), then that is the fault of preachers and teachers, not of any arrangement of letters.