The debate over this question is ongoing, and the real question of the matter doesn't revolve around the meaning of the words which Gregory of Tour wrote in the second book of Libri Historiarum. His words in Latin simply don't support the translation "Bow your head, proud Sicamber!" (More about the Latin shortly.)
On the contrary, the debate revolves around the chronology of events and the intentions of Gregory of Tours. In his article, "Gregory of Tours and Clovis", Ian Wood wrote the following:
The continuity of the debate is in itself an indication that, pace
Tessier, the chronology of Clovis's reign is an issue of significance.
Indeed the chronology affects, or is affected by, one's interpretation
of the career of the Frankish monarch. At the same time the account of
Clovis in Gregory of Tours is intimately linked to that historian's
purpose and his view of history. It is, in fact, unfortunate that the
historiographical issues raised by the Decem Libri Historiarum are
usually kept separate from the historical problem of Clovis's
chronology.
I'm sure that there are a number of interesting questions involved, but I'm going to avoid them because I believe the matter goes beyond the purposes of this site.
To address your question about how an inaccurate translation could have prevailed for so long, it's hard to say. My guess is that Clovis' many defenders wanted to avoid the difficulties involved, but that's just my opinion.
One of those who addressed the question of the Latin was Jean Hoyeaux. In his article "Le collier de Clovis", Hoyeaux affirms that the Latin word colla refers to the necklaces and not to Clovis bowing his head. He also provides historical evidence that the use of such necklaces was common at that time.
Concerning the Latin, Hoyeux says the following:
Mitis depone colla, Sicamber.
Le latiniste sera bien étonné d'apprendre que ce bout de phrase n'est
rien d' autre que le texte original du fameux «Baisse la tête, fier
Sicambre» par lequel saint Rémi salua Clovis, à Reims, le jour de son
baptême.
Cette façon de traduire a beau être traditionnelle, elle n'en est pas
plus défendable:
Mitis ne signifie pas fier, mais humble.
Depone (dépose) ne s'est jamais traduit par baisse.
Quant à collum, si primitivement il veut dire cou, par extension
ornement de cou, collier, l'emploi du verbe depone empêche qu'il soit pris dans le sens de tête, surtout au pluriel. Pourtant, ce
latin n'est pas difficile et le premier écolier venu lira facilement:
Sois humble, enlève tes colliers, Sicambre.
Translation:
The Latinist will be very surprised to learn that this end of sentence
is nothing other than the original text of the famous "Lower your
head, proud Sicamber" with which Saint Rémi greeted Clovis, in Reims,
the day of his baptism.
This way of translating may be traditional, but it is no more
defensible:
Mitis does not mean proud, but humble.
Depone (take off) is never translated as lower.
As for collum, if it originally meant neck, and by extension neck
ornament, collar, the use of the verb depone prevents it from
being taken in the sense of head, especially in the plural. However,
this Latin is not difficult, and the first pupil to come along
will easily read:
Be humble, take off your necklaces, Sicamber.
