Hello. I have noticed that German people sometimes add "-n" to the end of a noun in the accusative case (only for certain words, not in general). I want to refer to this fact in an amusing way. Can I say that it is a language phenomenon? Thank you.
Changing a noun's form because of its case is called "declining the noun" in grammar. The noun form for this activity is "declension".
You might be able to make a pun with other meanings of the verb "decline", but most people would not understand it. This meaning of "decline" is used when we study other languages that do it more than English.
I suppose the 'correct' name for an additional ending like that is an inflection - from Lexico (Oxford Dictionaries)A change in the form of a word (typically the ending) to express a grammatical function or attribute such as tense, mood, person, number, case, and gender.
By and large English doesn't use inflections much whereas German and all the other languages I've learned do. But that's really a quirk of English rather than the other way round, and I can't really think of an amusing way of expressing that idea.
I think the quirk here is not that German has inflections, but that only a very small number of nouns get this particular inflection. It definitely qualifies as a quirk of German in my opinion, and I would say that’s an amusing enough way to put it.