@AmigaMaster: I'm not the writer, so I'll give my best guess trying to answer your questions.
I know a certain snake that was charming too. Repent!
Ich kenne da auch eine bestimmte Schlange, die war mal charmant. Kehre um! ////Had some problems translating this, really needs a better translation! Which snake is meant?
I would assume it's referring to the snake in Adam and Eve... but I might be wrong.
It used to finish the Fjord run in 12 nautic miles, you know.
Sie hat einst den Fjordlauf in 12 nautischen Meilen beendet, müssen sie wissen.////Anyone knows what a Fjord run is? The name 'Fjordlauf' exist in German but I don't know what it is so I translated 1:1.
"a long, narrow, deep inlet of the sea between high cliffs, as in Norway and Iceland, typically formed by submergence of a glaciated valley." - According to Google translate.
The "run" is probably just a competition.
Bye bye.
Tschüss.////Bye Bye typically is used when you know someone pretty well. Between two strangers, you would use Good bye. I translated into German bye bye 'tschüss'. Good bye would be 'Auf Wiedersehen'
I'm not really sure you need help with this one. Seems perfect to me.

You mean, open that person with a knife or something?
Du meinst, ich soll die Person mit einem Messer oder ähnlichem öffnen?////Open person? :-) Looks like this is unused parts for Relic of the Viking?
The wonderful life of source code. full of unfulfilled ideas and stuff.

'Forget this folly and remove one from %SIDENAME%.', He said to them.
'Vergesst diesen Wahnwitzigen and verringere %SIDENAME% um eins', sagte Er zu ihnen.////Unable to translate without some creative rewriting due to the different articles "das" (heart and verb) and "die" (soul)
Be creative.

I can't tell you how to translate into german. I know too little of it to even be helpfull here. selmiak?
You now have access to the following extra features :
Du hast jetzt Zugriff auf die folgendes Extras:////Would you like to talk to the player personally, like a buddy (then it is "Du") or from some distance, then it is ("Sie"). I would recommend "Du"
I think this is up to you to decide. But I know the dilemma.
I have better things to do than rehearsing my course.
Ich habe besseres zu tun als meinen Unterricht zu proben. ////Is this correct? Indy wants to test (rehearse) his student course?
I don't remember this line in the demo (oh, probably for the old demo?), but I would guess it's probably intended as "This is a pointless discussion" "he already knows that"... or something to that effect.
Someone left that artifact on my desk before the class.
Irgendjemand hat das Artefakt auf meinem Schreibtisch im Klassenraum liegen gelassen.//// On teh desk inside the class room? (That's what I translated)
Yes, I suppose so. Someone went by dropped the "item" on his desk, inside the classroom, and then left. I don't think people are suppose to be able to go by his office when he's not there. He's got a secretary for that.

... This stone artifact was discovered in my class dig.
... Dieses Steinartefakt wurde in meiner Schulausgrabung entdeckt.////Class dig = school dig (excavation)?
Class dig = his class, the one he teaches at the College. They probably went on a field trip and do a dig (excavation) to learn and look for ancient artefacts. Of course, if could just as well be when he was a student and went on a class dig... but I'm assuming it was while he was the teacher.

Let's start cooking, BITCH.
Lass und was kochen, Schlampe. //// Cooking means really to cook something to eat? Did not find any other meaning in english urban dictionary
Well, one can also cook drugs. But can also be an expression for "let's get to work", 'let's do it". I don't remember where the line is used, but I'm guessing it's meant as the later and not really referring
to cooking a meal or food.
There is already a WaitNoBlock going on.
Es gibt schon einen WaitNoBlock////
Internal AGS command 'NoBlock'
Yes, this is internal code use for debugging. Doesn't need a translation.

just delete the translated line.
It fits.
Er passt. ////What fits where? Or is it commonly used? I assumed it applies to the small key, so it is "Er" in German
Yes, probably refers to the small key. But you might want to play and look for the line in game to be sure.
*Crrrrrrr*
*Knarrrrr*////Which noise should this be? I guess shields (some wooden noise) - I was not able to get this text from my playthrough and save games
No idea where this one is.
I don't see the mop.
////Wish mop or something else? Also did not find where to trigger this.
Likely another lost line from old source code.
Lines
//// Which Lines?
Probably inside code that doesn't need to be translated.