Just about any contract drafting or legal style guide you pick up will tell you to avoid using word–numeral doublets:
E.g., instead of:
He left € 800,056 (eight hundred thousand and fifty-six euros) to his cat.
He left € 800,056 to his cat.
Although the first version may feel safer and more legal, it leaves room for someone to inadvertently add in ambiguity in the form of a discrepancy between the two forms of the number. If not at the initial drafting stage, somewhere down the track when changes are made. It also adds redundancy — another pet hate of drafting style guides.
According to Garner (in this book), the doublet was originally used as a safeguard against fraud. As the numerals by themselves could more easily be changed, you wrote the figure out as well. It was also useful when carbon copies were used a lot, with the written-out numbers being easier to read on the copies.
Nowadays, unless you’re writing out a cheque or making carbon copies, there is little reason for using word–number doublets any more.
Obviously for translations, the risk of inconsistencies is even greater because there is at least one more person in the process, the translator, who also may introduce a discrepancy between numeral and word.
When translating, I think it’s a good idea to just use the numerals in the translation, even when the source text also writes the number out. At the very least, you’re using recommended modern English style.
Of course, by the time you get the document, a mistake may have already worked its way into the document, in which case, if the mistake is not obvious, you’ll just have to reflect it in the translation or sort it out with the client.