Deutsche Bahn ticket offices can sell reservations for TGV/AVE Barcelona-France. This summer it was possible also for trains that run only in Spain (trains run by RENFE) interrail.eu is not more able to, DB maybe the same. An other option for trains in Spain is the RENFE call center where you can reserve the reservation and take it at the train station in Spain up to some hours before departure.

Thanks Angelo, I was at the ticket office twice already telling them that and they insist that they cannot do it. 
Problem is I won’t be in Spain to use the pick-up service from Renfe. Because I board the train in Lyon. 

For the TGV from France it is not possible to get it trow RENFE call center, only for trains in spain!

Yes I know, some people at DB offices are not the best. Eurail Aid Offices | Eurail.com check one of the aid offices (like Berlin Hbf counter 12) out in Germany, there are people that are good at reservation booking. 

After 3 hours on the phone I had a really nice woman attending me and she accomplished to book all of the reservations. Finally!!
If anyone else is trying to make the reservations, to shorten your wait you will need to call 0305512 and than waaaaait! You will need a Visa oder Master Card to purchase it via phone and it can only be sent to you by post - so it takes time!
Thank you Angelos for your reply!

Is this a Deutsche Bahn phone number in Germany? DB tickets can also be collected at DB ticket machines or the ticket office if you have the order number.

For which exact train was this?

Yes it's Deutsche Bahn International. If you call 0302970 they will pass your call there. 

Apparently collecting tickets at the DB machines is no longer possible, at least this is what the lady told me, they seem to have stopped that service a couple of weeks ago.

It was the RSC9743 from Lyon to Barcelona on 2nd of december. She also managed to book reservations from Barcelona to Valencia (EURmed), Alicante to Barcelona (Euromed) and Barcelona to Montpellier (also Renfe SNCF Corporation) 

But she also said it is always very complicated and doesn't work at all times, depending also on the contingent of pass holder seats. 

Hopefully that helps someone also struggling with this.. 

Cheers!

Thanks @Mitri, this is very interesting! There are indeed limited quota for pass holders, especially on the TGV/AVE services France - Barcelona. Domestic Spanish services should be available until the train is physically full (which does happen sometimes). Collecting at a DB ticket machine is still possible, according to the DB website, but maybe the website is lagging behind. That would be very unfortunate; I'll look into this further.

@Nanja, apparently DB can still (or again?) book domestic RENFE reservations. Shouldn't the Interrail reservation service then also be able to do this?

@rvdborgt, not necessarily. We get our database source from a different provider than DB (as they are using their own database and thus not 100% based on the source we use). Most of the time, the timetable information and option to book is the same, but there are some exceptions with smaller railway companies since their data is not always updated. We will further investigate the Spain- France connections being bookable via DB. A ticket has been created for that. 

Regarding domestic RENFE reservations not being available in both our reservation system and in theirs is currently under negotiation. At this point we cannot confirm whether these reservations will become available in our system. 

Unfortunately, this number (+49 30 5512) is not reachable from abroad. I just tried it. The general number +49 30 2970 works.

Do you know if this is a regular phone number if you are calling from a country other than Germany? So without extra costs such as a premium number?

Anyone who also knows how much DB BAHN charges to send a ticket by post? 

+49 30 2970 is a regular phone number in Berlin. Normal charges apply.

DB charges EUR 5,90 for sending tickets by post; from EUR 250 there is no extra charge.

@Sven  you can try to call +49 302970 and then ask to be put on line with DB International. That’s how I did it and it worked (after spending another 1,5h hours wainting in line!)

The price of 5.90 euros is already much less than the 9 euros (!!!) that Interrail asks, so that's nice.

However, if you have to wait 1 hour or more on the phone, it will be expensive compared to calling the premium number of B europe.

Excuse me, what exactly do you mean by "ask to be put on line?"

Is there such a long wait every time you call this number from DB?

On the international booking website of Deutsche Bahn (only available when you book Tickets via bahn.de that doesn’t run throw DB Tickets, like Italy, Nightjets, France, ect. International Booking (international-bahn.de)), I found this information about how to contact DB International:

Your contact for all questions concerning international tickets is our service center for international bookings via phone number: +49 6172 913731 (standard fees for calls to the German landline network apply), or via e-mail: service-international@bahn.de, operating hours are: 8 am to 8 pm Monday to Friday and from 10 am to 5 pm (Central European Time) on Saturdays, Sundays and public holidays.

I don’t know if it is the same call center like the one that contact Mitri.

@Sven I mean you call the normal hotline and then explain that you try to reserve seats on a foreign train, then they told me that they gonna pass the call on to DB International who are able to do the booking. She told me it was the number I posted above. I also tried the +496172913731 and couldn’t reach anyone. 
I don’t know if it’s always such a long wait, but I think you should be expecting it to take some time. Maybe in the early morning oder evening it’s better.. but I don’t know. 
Good Luck

@Angelo On (international-bahn.de) website I dont see any option to book using interrail pass? Can it be booked online using interrail pass at all

@Mitri If I book the tickets via db call center then can it be only delivered in Germany?

Is there any way to get it online or on email for trains from spain to france?