I totally agree with your point about the necessity to keep the language "clean".
My intention is not really about keeping it "clean", it's more like keeping it "sane" ( = a language which is nice to speak / write).
I've no problems with new words being derived from other languages if there's no common translation available (yet). This has happened in the past and will keep happening. The only thing we should avoid is any excessive influence, like English is currently the most dangerous language. I can remember that France has an official association in place to do this job to keep too many English words in check.
The first one is worse for me, and it lies in
Surzhyk, as we call it in Ukraine. When grammar rules, phonetic features, sentence construction structure from two or three different languages are mixed. When the words of one language are pronounced in the manner of another, with intonation, stress, characteristic of another language. It sounds terrible, unnatural, illiterate and disgusting. We have a lot of this in Ukraine, because the Russian language and culture were planted here for centuries, and people forgot their Ukrainian language because of this. I believe that what you mean by Denglisch is closer to Surzhyk, and I can not agree more that thees phenomena must be erased from any language.
Okay, I don't know this for German maybe it's because Ukrainian and Russian are very similar.
But interesting story.
However, I also can think about the second case. When some of the words from another language are "borrowed" and begin be used in other one. For example, such words as selfie, smartphone, like, laptop are commonly used in Ukrainian language.
...
And it is impossible to stay aside, not for the separate person, nor cultures. There are no more separate, authentic, closed cultures and languages, because the impact of globalization is very visible and strong.
This has happened in the past as well and for German, some words are
derived from French, for example and also for words, where no German word existed before. This is totally acceptable, if there's no direct German translation possible, considered that these words are now pronounced in a German way, that they get common in our language and such cases are enriching our language because such a word didn't exist in our language before.
Like you've said: Selfie, Smartphone or Laptop (all words capitalized as we are writing German) are words, where we don't have an appropriate translation in German as well. Maybe there will be one at some point but there's no one yet.
So, a simple rule regarding unnecessary Denglish: if there's a (common) german word existing, we should choose it of course. If there's
no (common) German word existing it might be appropriate to "borrow" a foreign word, if it's fitting into the German language and can't be translated in a common way. For example, for "Blockchain", we don't say "Blockkette" (which would be a currently non-existant translation, still people would understand it but it's not common).
It might change but I wouldn't call "Laptop", "Blockchain" etc. to be "unnecessary Denglish" right now.