Gather the diplomas, complete transcripts, any professional certificates, the signed offer, and the employer's duty letter first. Seeing the full set together shows which documents genuinely need translating and which terms have to stay consistent across all of them.
Order the whole set once, not piece by piece
Translating documents one at a time invites different renderings of the same program name from different hands. Assemble everything, decide the vocabulary, and give the translator the full set with a short note on the terms that must be kept stable. Keep the originals available as well, since a reviewer may want to compare the translation against the issuing institution's own wording.
Build the index before the first order goes out: one row per document, showing the issuing body, the date, the language, whether an original can be produced, and the term it introduces. Rows that share a term are the ones a single translator should handle. Hypothetical example: a research assistant at a post-secondary institution holds a course attestation, a thesis attestation and an employment letter, and all three name the same laboratory in slightly different ways, so deciding the English form once removes three separate objections.
Ask the employer to hold the offer letter until that vocabulary is fixed, because reissuing a letter takes longer than correcting a translation.