Are existing posts automatically translated when adding a new language?

If I add a new language to the site, will the plugin automatically create and translate all previously published blog posts into that new language? Or do I need to manually trigger translations for existing content?

After adding a new language, new posts will be automatically translated. However, for existing posts, you'll need to trigger them manually.

When you do so, it'll be wise to temporarily disable the other languages, so you don't trigger those translations again.

In other words, keep only the new language enabled, trigger the translations, and when finished enable all languages again.

You can do so going to the Settings, under Plugin Configuration > Languages & Providers, and setting Translation provider per language option to None (Disable Translation).