Translation lands the meaning and loses the register

The remaining errors are social, not lexical

For common language pairs the vocabulary problem is largely solved. What comes back means what the source meant, in sentences a native speaker would accept as grammatical. The failures have moved to a layer that is harder to see: how the text positions the writer relative to the reader.

Languages encode that relationship differently and unavoidably. Some require you to choose a level of formality in every second sentence through verb forms or pronouns. Others carry it in vocabulary and sentence length. A translation that is correct in every word can still address a client the way you would address a schoolfriend.

The consequences are not evenly distributed either. Excessive formality reads as stiff and is quickly forgiven. Excessive familiarity, in a business context and in the wrong language, reads as disrespect and is not, which makes the safe direction to err in obvious once you know the axis exists.

Specify the relationship, not just the tone

Asking for a professional tone is close to useless, because professional is realised completely differently across languages and industries. What works is describing the situation: who is writing, to whom, in what capacity, with what history, and what the reader is expected to do next.

A first email to a prospective supplier, a reminder to a customer whose payment is overdue, a note to a colleague of ten years — each of those maps onto a different set of forms, and the mapping is what a translator with local knowledge supplies. Supplying the situation lets the tool make that choice deliberately rather than by default.

Where a language offers an explicit formality distinction, name it. Say which form of address to use and be consistent about it throughout, because inconsistency within a single message is more jarring to a native reader than picking the wrong level and holding it.

Some things should not be translated at all

Idioms are the obvious case. Rendered literally they become nonsense, and rendered as the nearest local equivalent they can import connotations you did not intend. The reliable move is to rewrite the idiom out of the source before translating, saying plainly what you meant.

Humour, wordplay and anything depending on rhythm belong in the same category. These are rewriting jobs rather than translation jobs, and the honest instruction is to convey the intent in whatever way works in the target language, accepting that the result will not be a translation in any strict sense.

Then there is material that must not drift at all: legal clauses, safety instructions, dosage information, contractual obligations. There, fluency is not the goal and a stiff literal rendering reviewed by a qualified human is the correct product. If the text creates a legal obligation, the translation should be done by someone who can be held responsible for it.

Conventions are local and are not linguistic

Correct language and wrong document are entirely compatible. Address formats, the order of names, how dates are written, what a business letter opens with, whether a first name is acceptable, how much throat-clearing precedes a request — these vary by country and are conventions rather than grammar.

A translated text that ignores them reads as foreign even though every sentence is fluent, which is often the specific impression you were trying to avoid. Asking explicitly for local conventions helps, but this is the area where a native reader’s five minutes are worth more than any amount of prompting.

The same applies to what is polite to omit. Directness that reads as efficient in one business culture reads as abrupt in another, and the fix is usually structural — a different opening, a softer request form — rather than a matter of finding gentler words.

Round-tripping is a weak check and worth doing anyway

Translating the result back into your own language catches gross errors: a negation lost, a number changed, a clause dropped. It is genuinely useful for that and takes a minute. What it cannot detect is register, because the return trip discards exactly the formality information you were trying to check.

A better check, where you have access to one, is a single question to a native speaker: does this sound like the kind of message it is meant to be. That question is answerable in seconds by someone who could not have produced the translation themselves, which makes it unusually cheap to obtain.

For anything with money, obligations or a first impression attached, the honest position is that a machine translation is a good draft and a poor final product. Use it to get ninety per cent of the way in a minute, then buy the last stretch from someone who lives in the language.