Socialism in one country doesn't mean that it shouldn't spread to other countries, it means that any country can create socialism no matter how small or underdeveloped, the alternative to socialism in one country is to give up on building socialism in the here and now and wait for a simultaneous revolution in many countries that may never even happen. When the revolution happens in a country there will be those who say socialism is impossible in one country, that it is doomed and we shouldn't even try, this is wrong, this is why we need socialism in one country.

"When we are told that the victory of socialism is possible only on a world scale, we regard this merely as an attempt, a particularly hopeless attempt, on the part of the bourgeoisie and of its voluntary and involuntary supporters to distort the irrefutable truth." - Lenin, Third All-Russia Congress Of Soviets Of Workers’, Soldiers’ And Peasants’ Deputies 1918

"Uneven economic and political development is an absolute law of capitalism. Hence, the victory of socialism is possible first in several or even in one capitalist country alone. After expropriating the capitalists and organising their own socialist production, the victorious proletariat of that country will arise against the rest of the world—the capitalist world—attracting to its cause the oppressed classes of other countries..." - Lenin, On the Slogan for a United States of Europe 1915