First, almost any contract would include a force majeure clause which allows termination due to a war (among other acts).

Failing that, the German government could just change the law to disallow the contract. Russia could ask for a binding(ish) decision from the international community, but this is one of those situations where I doubt the decision would go in their favor even if they were in the right legally.

According to Ukraine's energy minister and an economic advisor in this article[1], gas is paid for in Euros and Germany could block the transfer to Roubles.

> We propose that the Europeans continue to pay for pipeline-delivered gas and oil, but that the euro-denominated accounts of those Russian pipeline operators be frozen in terms of withdrawals. If properly implemented and enforced, this would mean that all gas customers continue to meet their contractual obligations, while through force majeure the EU creates what would be effectively escrow accounts for the duration of the war.

[1] https://www.newstatesman.com/world/europe/ukraine/2022/03/vo...

Russia breaks its legal obligations all the time.

...and if Germany didn't honor that contract? who's going to litigate?

Possibly, but without knowing what the contract looks like there's no reason to assume it doesn't have break clauses like most contracts.

>legal obligation

something tells me that Germany doesn't care about that right now.

Isn't Russian under a legal obligation to not invade their neighbor and murder it's people and cities?

I'm sure Germany would welcome a Russian dispute in front of the WTO.