I'm going from Seattle to Karlsruhe. Since the flight to Karlsruhe from Seattle is really expensive, I am looking for options to arrive to Karlsruhe from alternative airports (Frankfurt or Stuttgart) and then find alternative forms of transportation.
Any suggestion on which airport and form of transportation? Also, if I could buy my tickets once I get there instead of in advance, it would help a lot, since it's my first time to Germany and unfortunately I can't speak German.
Stuttgart is a regional airport. You will find domestic flights going from Frankfurt to Stuttgart, but it generally isn't worth the effort to look them up for such a short distance. There may be direct flights from the U.S. into Stuttgart, but again, they aren't worth the effort to look them up. It's pretty much the same as with Karlsruhe Airport.
Book a flight to Frankfurt Airport, then take the train to Karlsruhe. Frankfurt Airport has a long distance train station with direct trains to Karlsruhe. The train ride is about 90 minutes, trains go each hour and cost 30 to 60 Euro, depending how much in advance you settle on a specific time (or not at all).
You can book tickets at http://bahn.de/en, from is Frankfurt(Main)Flugh, to Karlsruhe Hbf (or whichever bus/tram stop you want to go to in Karlsruhe). Or use one of the long distance travel ticket machines at Frankfurt Flughafen station, they also have an English menu. Ticket counter is also an option, they should be able to talk to you in English.
It's a good idea to look up the train and bus/tram schedules at bahn.de even if you don't buy a ticket online. They also have all local bus/tram schedules in Germany.
It is probably worthwhile to look into getting a Rail&Fly ticket. The general ideas are as follows:
Unless you are travelling a short distance, these tickets offer a very cheap price with incredibly huge flexibility so even if things go incredibly wrong with your flight, you don’t have to worry about losing the value of your purchased train ticket.
They also allow you to opt for the airport which has the cheapest/most direct flights (which is usually Frankfurt). Most major German airports have built in Train stations and since Germany has high speed rail links (up to 300 km/h).
Germans generally have good English, especially in industries which often interact with foreigners such as rail and air transport.
Definitely Frankfurt since the train station is in the airport. The airport actually has two train stations: one for regional trains (S-Bahn) and for regular long distance trains (Fernbahnhof). You definitely want the second one (Fernbahnhof). Every two hours there is direct train, that doesn't require changes. Tickets can bought at www.bahn.de and it's substantially cheaper if you buy them two weeks or so ahead of time.
There are currently only two airlines that go non-stop from SEA to FRA which are Lufthansa and Condor. Lufthansa will run you around $1300, Condor can be as little as half of that but they don't fly every day, so you have to have some flexibility in the dates. "normal" flights with a single layover should be around $1000. All prices are mid week, mid February.
A note on German train booking, if for some reason you are not using rail&fly (whose availability and price is dependent on the flight carrier you choose): you get two prices for each train, "fixed" and "flexible". Fixed (which is a condition of almost everything offered as "savings fare") means that you are buying a ticket for a given train and if you miss that train for whatever reason (except by fault of the Deutsche Bahn itself) you have to buy a new ticket to travel later. The Flexiticket is valid for any train travelling that day, and in connection of the uncertainty of intercontinental flights and the stress of long travel to an unfamiliar destination, I would consider it money well invested.
As for booking from home, you can buy the train ticket under bahn.de, payment by credit card is accepted, and you are obliged to carry with you a printout of the ticket. As said above, compare prices with Rail and Fly first - it seems that you will be paying around 40 Euros from Frankfurt with the ICE. This is the price for one direction, you don't get discounts for booking the return journey at the same time.
For the preferred order of airports: the other answers already explained why Frankfurt is by far the best choice. If for some reason you want to compare to second-best alternatives, then Stuttgart (1-2h), Strasbourg (1-2h), Paris (2-3h) and Zürich (3-4h) would come into consideration, in order of reduced time spent on board of a train. For basically all of those, you can find connections with only one layover somewhere other than Frankfurt, so if the options you have through Frankfurt are very inconvenient for you, you can shop around to see what happens if you fly to one of those. Smaller airports which are physically closer are unlikely to be a better choice because of more layovers or worse train connection.
