1. Rap is mainstream and independent of race, ethnicity or country. Since the rapper is white, this is really not an issue about language but about punctuating the lyrics. For example, Epic Rap Battles (
www.epicrapbattlesofhistory.com) are a comical use of the musical format. As Alcove Audio tried to emphasize, rap comes in many "flavuhs". If you are selling this you don't need to create lyrics. A producer can hire a musician to create that. You can if you feel comfortable doing so, then go ahead.
Code:
JARED
So here's my newest
song.
Jared raps, gesturing and body moving as his friend's head
bobs with the rhythm.
If you're putting the lyrics in your script, it's usually good to show that it is sung by listing the title and putting the lyrics in italics.
Code:
JARED
So here's my newest
song.
["Dollaz"]
[I]Yo, lis'un, gonna tell ya straight,
elaborate/ no big exec will be a
cryin' when your lyin'/ face down
in the street a dyin'/ ...[/I]
Jared raps, gesturing and body moving as his friend's head
bobs with the rhythm.
Rap is here to stay, though that doesn't mean it won't evolve.
2. I think if you introduce how the protagonist comes to have the device along with its shortcomings to the audience, it's a non-issue. Science fiction is about plausibility without necessarily explaining details. The details of how a "transporter" worked in Star Trek were only vaguely suggested. It's a convenient plot device. When it fails, it's a plot device. You just need to outline what the expectations of its behavior is for the audience.
Code:
James looks about anxiously as he sets the briefcase on the
bed.
Jared watches him. His eyes narrow and he studies James.
JARED
Is it gonna explode? Your pacing
is making me seasick.
JAMES
This isn't supposed to leave the
lab. I was talking.
JARED
To that hot brunette?
James smiles and shrugs.
JAMES
Long story short, the lab got locked.
I had no choice but to bring home
the V.R.P.
JARED
V.R.P.? Virtual Reality Portal?
James opens it up and pulls out something that looks like
braces.
JAMES
Vocal Redux Peripheral. It links to a
phone app to replace a vocoder. The
prototype uses vocal impulses and
comes with programmable voices.
James pops it in his mouth. He pulls out his phone and
fiddles with an app.
JAMES
(Darth Vader voice)
Jared. I am your Fah-ther!
Jared smiles and shakes his head.
James pulls it out and puts it back in the suitcase.
JAMES
I'll just be happy when it's back in the
lab. If they knew I had this, I'd be in
deep shit. Totally experimental.
James heads into the kitchen.
Jared turns back to work on his lyrics. He pauses and
glances back at the bed.
3. Lots of science fiction seems less relevant 10-20 years after the fact. That's no reason to not include it. It doesn't change the relevance of the story. I'd say go for it.
4. It's very hard to sell shorts. And I don't think you have enough of a story to make a feature to be honest. I don't see anyone buying it in the US. Use the language that will be appropriate for your actors. If you're filming with a Hebrew or Russian speaking group, use that language. That doesn't mean you can't use rap. I think the ethnicity of the music producer and his friend can be dependent on the actors available.
As for scriptwriting, the conventions were created for English scripts. It can be adapted. Use the the margins mirrored in Hebrew. And instead of caps, use bold. Since you are already producing/co-directing another film, follow what you're doing.
Again, I wouldn't put a lot of attention into the lyrics at this point. They aren't the focus. The story of the misuse of a stolen technology should be the focus. Good luck.
But the're kids. That's what they were supposed to do. Nobody expects kids to sound hardcore and brutal. To my opinion, they were meant to be hilarious 
