Impact second language on SEO

My website is in Dutch and primarily for Dutch visitors. However, because occasionally people from other countries do visit, I have added an English version. I read several disturbing posts on this forum from people who’s website didn’t show up in Google anymore in their default language. From a SEO perspective, only the Dutch (default) language version really matters, in my case.

So my question are:
1) If you’re not using the SEO Pack Addon, does adding a second language version affect your search engine results in the default, primary language? If so, there anything you can do to prevent/minimize this impact?

2) Does using the SEO Pack Addon improve search engine results in the default language? And if so: have issues like websites appearing on Google in secondary instead of default language been resolved?
(https://wordpress.org/support/topic/search-engine-results-is-not-showing-in-my-default-language/, and https://wordpress.org/support/topic/duplicate-urls-in-search-console/)

The page I need help with: [log in to see the link]