Pre-booking a visit to the Reichstagsgebäude in Berlin

If it's already fully booked then that will likely not change. It might be possible that people cancel their appointments in advance (though I do not know if that is actually possible). If that happens, slots will become free¹. You should be checking regularly if there are free slots in case that is an option, but their website does not say anything about cancelling your appointment, so I doubt it.

Hoping for a free slot when you're already there while it's booked now is most likely going to be futile. Don't get your hopes up.

What Andrey said in his comment is correct according to this flyer (in German).

The easy-to-use audioguides, which you can obtain on the roof terrace, are available in eleven languages: German, English, French, Spanish, Italian, Polish, Portuguese, Russian, Turkish, Dutch and Chinese. Special audioguides will also be available for children, for people with disabilities and in easy language.

You cannot get the audio-guide while the dome is closed.

If you want to visit a Plenarsitzung (an assembly where they make actual politics) you need to translate for your friends. There are no official translators.

Depending on how many friends you are bringing, you could ask for an individual guided tour in English or another language. This is mentioned on page 11 of that flyer.

Hausführung für ausländische Besuchergruppen
Auf Anfrage sind Führungen für Gruppen ab sechs Teilnehmern in den Sprachen Dänisch, Englisch, Französisch, Italienisch, Norwegisch, Polnisch, Russisch, Spanisch und selbstverständlich auch in Deutsch möglich.

It says:

You can request a tour for groups of at least six people in Danish, English, French, Italian, Norwegian, Polish, Russian, Spanish and German.

You need to apply for those in writing. The number of participants cannot exceed 25, but it doesn't say anywhere if they mix the groups up.

You need to write to this address or use the website.

Deutscher Bundestag
Besucherdienst
Platz der Republik 1
11011 Berlin
Fax: +49 30 227-30027
www.bundestag.de/besuche/formular.html

Include the type of event and number of people you want to register for, a date and time as well as up to three alternatives in case they are already booked. Also your name and phone number.

There are also tours in German of the various other parts of the government building complex, like the Paul-Löbbe-Haus.

^{1) This is the way to go for getting an appointment in almost all Berlin state/city offices like for registering an address, because they are horribly overbooked for several months in advance}

As far as I remember, you can also get a pass to get inside from the counter outside, I have seen long queues there but generally it looked mangable to get the pass.

Disclaimer: I always booked online so haven't used that counter but I am pretty sure its possible to get pass from that counter. It might have changed in recent years orb due to recent attack but still worth exploring.