The requirement to pay for the gas in rubles is a desperate decision by Russia in order to try to maintain the value of ruble. However, I can't imagine the contracts not precising the currency, so if Russia stops the gas delivery, it will be Russia breaking the contract, not Japan or any other country, for not paying in rubles.
Many countries already responded that they will not pay in rubles: Poland, Germany, France, just to name a few.
Russia is being infantile and that further damages the future of business with this country.