Working with literally dozens of Sitecore clients around the world - and helped get content from all the largest and many small translation firms - I can confirm the inefficiency of the on-site translation attempt, that is, at Sitecore, I liken it to asking an electrician to come and reinstall your home, but when they get to their toolbox from the truck, you will tell them, "No, you have to do it manually."
The best way to manage anything more than a page or two content for translation is to easily export it. Deliver it to the LSP in the correct format (XML or XLIFF) and, when possible, automatically import it into your TMS. After the translation, the content should then seamlessly return to Sitecore.
You can program it yourself - but the pitfalls are non-trivial only on the Sitecore side. (If you want an intuitive user interface, scalability and all the features that suit the needs of the translation). Not to mention the problems of connecting to LSP systems. (For example, who knows the relative merits / risks of using the Nexus SLD connector versus their CTA to connect to the TMS?)
As mentioned above, there are commercially available solutions that meet all these requirements and much more. Therefore, if you even have a modest amount of content and want to send it to any translation provider of your choice, I would love to discuss how we can help.
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