Date format in another language
The Drupal way of handling and translating Dates is through Configuration &gt; Regional and language &gt; Date and time formats, as Hudri mentioned in his comment. 

The same date and time Pattern between languages

What you should do depends on what pattern you want your date to be in English, if it should also be 'd F Y' as in Dutch. If yes, then you should edit the relevant Date format through the interface. Content types use Medium as default for the “Submitted by” so edit that to keep things simple. 

I expect you'll have the date and time related terms already translated into Dutch, so you'll get oktober and not October on the Dutch side. 

Different date and time Pattern between languages

If you want the Pattern of the same Date format to be different between laguages, for example you want English to be “10/16/2017” with Dutch “16 oktober 2017” you need to translate the Date format Pattern. 

What is important to mention is that you need to have the Configuration Translation module enabled to be able to translate Date format Patterns. This will add the Translate link to the Operations button drop-down: 



This enables you to have a different Pattern for different Languages under the same Date format. 

Don't forget to Flush cache if you don't see the changes on existing content!

Code manipulation

The format_date() is Depricated, more info: Date formats API changes, public function DateFormatter::format.