Backstory: Recently a client inquired about translating their website into Spanish. Not knowing Spanish, I looked for a few services and they exist but none really position themselves as copywriters. Your website will be a different language but it may not have appropriate sales copy.
The Idea: Website translation by a copywriter.
Deep Dive: You would charge a fee based on the number of words. The key value you would provide is expert translations by a copywriter. You would be able to judge metrics such as leads and sales pretty quickly. For larger sites, you may even be able to strike a bonus for revenue numbers.
The biggest hurdle is finding the correct copywriter. Niching down into one language at first will help. With the first client, you could split test the versions and see which one does the best. You may even lose on the first deal but you would have an interesting case study for the next ones.
Validation: There are already generic translation services.
Monetization: As mentioned above, you could charge per word. You could also offer bulk discounts for multiple languages as you expand.