Translation is complex. The basic conversion is relatively simply - it's all the edge cases and subtle differences that slip in and greatly compound the work.
Even sentence by sentence might not be enough - translation needs context of use.
what's your tech stack? Most popular commerce platforms are already ready for translation, it's easy to buy a pre-translated package for the non-customized portions of the platform and then you only need to worry about the wording you've changed.
We just finished our first translation effort. We used a service called localeapp.com that exported all of our prepped english language files to their service.
Then I made a request on fiverr for the work based on how many words we had in all of our files - I specifically mentioned that we needed them to do the work in an application instead of via a text file or other common use. A very competent native speaker with a tech background was the only respondent to our request that mentioned the localeapp and asked details about it. She did a great job (later verified by native speakers using our app.) We ended up getting our entire app translated for way less than anywhere else and have established a proven resource for updates as we make changes.