Easy export/import of data for Translation

1st as you figured out, WordPress does not offer any built in multilingual support for posts and pages.
2nd Themes, plugins etc. need a separate translation workflow than pages. But that is usually ok, since the theme does not change as much as the pages (normally).
To translate the themes I would recommend Loco Translate
https://de.wordpress.org/plugins/loco-translate/

For the posts, you already found two of the more common plugins. Personally I would not recommend qTranslate. I think active development has stopped and it stores all translations into one posts. This makes it complicated to change the plugin later. WPML is the most used one, but is not really open source and is mainly built for iCanLocalize. If your translation agency is iCanLocalize, then this is the best choice.

PolyLang is from my opinion the best solution. It’s completely open, but has plugins so you can work directly with different translation agencies or translate directly inside wordpress. My company Supertext also built the plugin Polylang-Supertext for it. So you can directly order your translations out of WordPress. This should cover all your requirements.
Let me know if you have additional questions.

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