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According to a local agency that specializes in authorized translations into English:
what pages (or all of them?) should be translated into English for the UK visa center?
In addition to translating the original, it is possible to translate relevant parts from other attached translations (like the stamp confirming the Russian citizenship in our case). The final decision on what is necessary for UKVI is at the applicant's discretion.
is it possible (advisible) to attach the English certified translation to the original of such a birth certificate (possibly damaging the Russian translator's stamp)?
It is possible to attach a new translation to the existing bundle using a different corner of the original.