Is it possible? From Germany to Portugal

Ja, yes, but not as easy or straightforward as f.e. from H to HH on always late DB. You also did not tell if you want the ´cheap´ way or the comfortable or what places in between to also visit.

What is best route from DE to FR depends on your hometown/starting point. Avoid Thalys, very high supplment to pay.

Best site (in /en/) that gives overviews on overland travel from UK/LON to all EUR countries is seat61.com. As the overnight train from FR/ES border on Atlantic side does not run anymore, in fact the only feasible way is roundabout via Barcelona-Madrid-then depends if you want Lisboa or North PT.

From MAD there is 1 daytime connection with short changes via Merida-Badajoz-Elvas-Entroncamento to Lisboa.

This means you have to overnite in either MAD or Barca (superfast trains run hourly Barca-Mad in 3 hrs).

Now about 60% of all Interrail newbees want to use the 1/day direct TGV Paris-Barca and this is a real bottleneck: you MUST reserve and there is also a high mark-up (supplement) so every day we have to explain that they have made the wrong choice. Also ALL trains in ESpana must be reserved and again the HiSpeed cost extra. Same-same for TGV in FRance. So try to go via Strasbourg and join the line to Spain somewhere south OR-in summer, the tipical old-style InterRail gathering place: the overnight train Paris-Port Bou (border FR/ES on Mediterranee), then local train (rodalies) to Barca. For general planning use the travelplanners of the railways in that country and break up in segments-otherwise you will still be steered into what is the fastest connection.

As this question has been posted many,.many times, just scroll on and read more posts. And read the general info on site re bookings/REServations and country info. Same-same on other InterRail helpsites: the old rail.cc is now raildude.com.