Man muss schauen

I can't tell how good it is translated, I don't understand French. But you made a lot of grammar and spelling mistakes.

I see. Could you point out some of them, if it is not too much trouble?

Okay:

"Schmutzigkeit" or rather "Schmutz"
(if you change it to "Schmutz", the next line has to start with "der" instead of "die")

"die sich vor uns dehnt"
I guess "dehnt" is the wrong expression here, propably "ausbreitet" or "erstreckt" is what's meant.

"Hinter den geschrumpften Augen" Does this mean "behind the wrinkeled eyes"? Then it has to be "hinter den runzligen/faltenreichen Augen"

"Über diese offenen oder geschlossenen Hände"

"Muss man zuschauen"
"zuschauen" is an intransitive verb, it means "to watch". I guess "schauen" or "blicken" ("to look", "to see") or "betrachten" ("to look at" (for a while)) would fit better.

"Man muß zuschauen
Alles Schönes, das existiert
Den grauen order blauen Himmel"
I guess "zuschauen" refers to all the following lines? I would use "betrachten" here instead.
The lines have to be
"Man muss alles Schöne
betrachten, das existiert,
Den grauen oder blauen Himmel"

"Den Freund, den du weißt treu"
Can't be expressed that way. It could be "Den Freund, von dem du weißt, dass er treu ist"

"Man muss
den Vogel im tiefen Wald hören,
Das Flüstern des Sommers
Das Lied das sich in uns selbst erhebt"

Thanks for your help, Sciera
I changed the text according to your advice, with a few exceptions.
They are:
"Ansehen" instead of "schauen", "blicken" or "betrachten" because I think "ansehen" expresses intentness more than the others.

"Alles Schönes"
The German has expressions such as "etwas Schönes" or "nichts Schönes".
So "alles Schönes" is also acceptable, isn't it?

"Den Freund, von dessen Treue du weißt"
Is this expression acceptable?

I guess it conveys the meaning better than the other verbs, but it can't be used with the prepositions/adverbs "hinter", "über" and "ferner". That is why uyou have to use "blicken", "schauen" or "sehen" in the first paragraph ("betrachten" doesn't work here either). My advice to use "betrachten" only refered to the second paragraph.

I didn't write that the whole expression is wrong. But it has to be "Alles Schöne", not "alles Schönes". That is because "alles Schöne" uses the same declination pattern as "das Schöne" and not as "etwas Schönes".

"Den Freund, von dessen Treue du weißt"
Yes, that works.

Oh, and you said you changed the text? I don't see any changes in paragraph 4 yet.

And you made a new mistake: "Hinter dem Schmutz"
"offennen" is written ""offenen", as I already marked in my first comment.

I also found another mistake I didn't see before:
"Ferner als die Stacheldrähte an der Grenze"

I don't get your point. Are you saying that "alles Schönes" is grammatically wrong? Is "etwas Schönes" grammatically unacceptable here, too? I don't know what the declination pattern is.

Yes, I have changed the paragraph 4.
Den Vogel im tiefen Wald
Das flüstern des Sommers
Das Blut, das sich in uns selbst erhebt

By the way, "Das Lied" was corrected to "Das Blut."

Why? muß is okay, isn't it?

Yes, "alles Schönes" is grammatically wrong.
It has to be "alles Schöne".
"etwas Schönes" is grammatically right.

About paragraph 4: Yes, I now see what you changed. But there still are mistakes in the first 3 lines.
The order of words doesn't work. "hören" has to be after "den Vogel im tiefen Wald". (it could also be written even later in the sentence.)
And "Flüstern" has to be written with capitalized F.
It has to be
"Man muss
Den Vogel im tiefen Wald hören,
Das Flüstern des Sommers"

No, it isn't. It was spelled "muß" in the "alten Rechtschreibung" (old orthography). But since 1996 the "neue Rechtschreibung" (new orthography) is being used, and this changed and simplified the rules for "ss" and "ß".
So now, because the "u" in "muss" is short, it has to be spelled with "ss", not with "ß".

Oh, I see that now you changed the verb of the first paragraph to "schauen". But you didn't adjust the rest of the paragraph to it. "schauen ... über" is used with Akkusativ.
So the first four lines have to be:
"Hinter den Schmutz,
Der sich vor uns ausbreitet,
Hinter die runzligen Augen
Und die müden Gesichter"

EDIT: And in the second paragraph I wouldn't use the word "schauen" but "ansehen" or, as I said, rather "betrachten". That is because "schauen" is an intransitive verb, that means it isn't used with any object.

Okay. Now I have fixed all the mistakes, I hope.
But how much trouble you have to go through in German once you make just one change in the text!
You have to change articles, adjectives and verbs and so on.
In Japanese, you just change the wrong word and it is okay.

@snorio: German is a very difficult language, especially in Grammar. We're lucky to have Sciera!!