There are a number of points to mention here:
Firstly. hreflang is not a ranking signal, and therefore it will not provide a ranking boost.
Secondly, Google will not auto-translate the pages because hreflang was implemented. You'll need to do the translation yourself.
Thirdly, you don't have to translate all pages of your website to Duth, only the ones that would make sense to translate. For example, there could be blog posts that would resonate with an English speaking audience, but not a Dutch one, so there wouldn't be a need to translate these posts. The opposite could be true too.
What hreflang does, is that it helps search engines understand which pages are meant to rank for which languages and in which countries. The actual implementation can get tricky if you have multiple languages and several countries and I would recommend looking into a few tutorials/documentation on this, including:
Hope that helps!