The editor is responsible for Quality Assurance.
The editor has to be a senior translator and he will review the translation for accuracy, consistency, completeness, grammar, style, fluency, and he will also proofread it before and after it is typeset. Either the translator or the editor (or both) will be specialized in the subject of the text being translated: technical, medical, legal, etc. We use for this purpose many professionals who are active or retired from careers in industry, health services, law, etc.
Languages are like living beings - they change over time and from place to place. France and Quebec speak different varieties of French. Spain and the Spanish speaking countries in Latin America speak several different varieties of Spanish. The same with the USA and the UK, and with Portugal, Brazil and Angola.
Languages also evolve over time, especially the vocabulary used in new fields, such as biotech, computers and the Internet. So we will, in certain cases, have to send the final translated text to the country where it will actually be used, and have our specialists review it there. Or the client may have knowledgeable personnel in that country who can review the translation, mostly for technical jargon.