Translated guest reviews on a holiday rental site

Your company runs a holiday rental site in 12 languages. Guests write reviews in their own language, so many readers cannot read most reviews on a listing. You want each review shown in the reader's language, with a link to the original.

There are 6 million reviews, and guests write about 20,000 new ones a day. A review is about 200 tokens long. Listing pages show reviews 10 at a time, and readers see about 30 million reviews a day. About half of those are in a language other than the reader's.

A hosted model translates well. It costs $0.50 per million input tokens and $1.50 per million output tokens, and a call takes 1 to 3 seconds. Guests can edit a review for 14 days after posting it.

When does each review get translated, where is the result kept, and what does a month of translation cost?