The economics genuinely changed
Translating a large site was previously a project most companies never funded. It is now cheap enough to attempt, which makes the interesting question what still needs a human rather than whether to do it.
Terminology is the first failure
Product names, industry terms and your own vocabulary get translated when they shouldn’t, inconsistently across pages. Supply a glossary of do-not-translate terms and enforce it — the naming consistency problem in another language.
Machine translation rarely produces nonsense. It produces something slightly off, consistently, which is harder to spot.
Marketing copy needs a rewrite, not a translation
Headlines, taglines and persuasive copy translate poorly because they depend on rhythm and idiom. Treat these as briefs for a local writer rather than as text to convert.
Anything binding needs professional review
Terms, policies, safety information and regulated claims carry obligations that vary by market. This is the professional-liability category where a plausible translation is a genuine risk.
Don’t publish what you can’t maintain
A translated site is a permanent commitment to keep every version current. Versions drifting apart is worse than one language done well, and it is the same trap as supporting a language you cannot review.
Get the technical signals right
Language declarations, alternate-language links and separate URLs per language matter for both search and assistive technology. Translation without those produces pages nobody can find in the language they wanted.