I've used, with plenty of success (ending around 5 years ago), both welocalize.com and lionbridge. We sent them .JS files, XML files, PDFs, and Word Documents.
Some things to look for:
Across multiple transactions with these [edit: localization] companies, they end up building a database of phrases, of which some companies might try to claim ownership over. Watch out for that when negotiating.
You're also most likely going to pay per word, so if you can figure out a good way to not send them sentences you've already had translated you can save some money.
Its also good to spot check some of the translations. These companies will basically post out the requirements and get freelance translators to do the job (in other words, they're managing all the UpWork work for you so you don't have to). We once caught a translator (NOT of the above-mentioned companies) located somewhere in Europe inserting some choice political statements about a specific biotech/GMO company we had as a client.
(editted for clarity - below the first Paragraph is general knowledge of the industry, not specific to the two aforementioned vendors)