In linguistics, a register is a variety of a language used for a particular purpose or in a particular social setting. For example, when speaking in a formal setting, an English speaker may be more likely to adhere more closely to prescribed grammar, pronounce words ending in -ing with a velar nasal instead of an alveolar nasal (e.g. “walking”, not “walkin'”), choose more formal words (e.g. father vs. dad, child vs. kid, etc.)
In writing or translating, a good writer/translator chooses appropriate register for the context, i.e. legal, financial, marketing, HR, fashion, media, etc.
Part of a copy-editor’s job is to check and correct register mismatches in translation.