In case the country is Germany, here's the last official statement from the German Embassy in London that I have seen: "We don't know". In the case of the Netherlands, getting UK citizenship means your dutch citizenship is gone. 

If you are an EU citizen who lived and worked in the UK for five consecutive years at some point and didn't leave the country for two years then you have indefinite leave to remain, you just need to fill out the right forms to get a document saying so. You'll be able to change this for free to May's new "settled status" document. And you need this document first if you want to apply for UK citizenship (no UK citizenship without leave to remain document), so I would do this with some slight sense of urgency to have it all lined up when you need it.