As a writer of fiction, you are not a linguist. You do not need to describe language in an objective way, in fact that would even hinder your purpose of telling a powerful story.
Character description in narrative fiction is not scientific. It has the purpose of telling us something about either the character or the person describing him or her.
Let's look at an example:
"Julia, come to the blackboard," Mr. Bergmann said in his Nazi officer voice.
We can imagine that the German teacher, Bergmann, not only sounds like a movie character, but is also very strict and would love to resort to corporal punishment if he could get away with it. Or we can imagine that Julia is an emo teen who finds all figures of authority equally evil, and just projects her own self-pity onto the really very nice native German teacher's accent.
In my example, it is totally irrelevent wether or not there is such a thing as a "Nazi officer voice", the only thing that counts is that there is such a cultural sterotype and that you can make use of it.
Here is another example, using not accent but sound of voice:
"Oh, honey," Linda purred.
Obviously, if Linda actually purred her words like a cat, she wouldn't sound sexy but creepy, but there is this convention of using animal sound descriptors (bleat, roar, etc.) or general sound descriptors (rumble, grate, etc.) as short hand for certain emotions or traits. Again, depending on the context of the example, the purring would tell us something about Linda (she wants to seduce someone or is pleased) or the person perceiving and describing her (who might be mistaken and misinterpret Linda's intention or state of emotion).
But we don't have to use existing stereotypes or universally agreed upon descriptions, we can make up our own:
John spoke with the slow accuracy of a blinking cursor.
In this case we describe the speed and rhythm of someone's speech to describe a mindnumbingly pedantic, self-alienated nerd – or some observer's prejudices.
I hope these examples make it clear to you, that /jə doʊnt hæv tu juz ˌsaɪənˈtɪfɪk ˌɑbdʒɛkˈtɪvᵻdi/ in narrative fiction and that in fact scientific objectivity would make your narration as flat and uninspiring as that blinking cursor.
A note on "harsh" German:
How a foreign language will sound to a person will depend on that person's mother tongue and his or her mastery of the foreign language.
To a person speaking English, maybe German sounds harsh. To a person speaking an African click language, it will probably sound different. The point of reference is important, so there certainly is no universally correct way of describing any language.
And if you learn a language, become more familiar with it, and eventually make it your own, the initial outside perception of that language fades and you will find yourself unable to characterize that language. Foreign languages are most strikingly dissimilar, if you don't understand them and only hear them as meaningless sound. If you know them, the meaning overlays the characteristics. I'm a native German speaker and have read, spoken and written English for about thirty years now. The two languages no longer sound different to me, or no more than my two hands differ from each other: they are not the same, and they cannot do the same, but they are the same and do the same – if that makes sense to you.
