For some countries, decades ago, certain spelling's for our country's name established themselves as the most used, in France - Roumanie, in England - Rumania, for instance. Lately, though, for the english speaking countries 'Romania' tends to become the common spelling. Brits are more stubborn, as we all know, but they'll eventually use 'Romania' spelling too. 'Roumania' must be a compromise for the time being
. Anyway, Romania is how it should be written. In Romanian language the first a is in fact an a with ^ accent but if one uses only the english alphabet then Romania is the closest both in writing and phonetically.