What country (sovereign) lies entirely within (inside) another country,
It is not an Enclave?
Two possible solutions
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Based on existing answers, it's:
Russia, which is inside Belorussia
or, if you prefer
in Belorussian, Русь inside Беларусь (using slightly glorious, less standard country name).
Alternately and more geographically,
Munich in Italian is Monaco, and it lies completely within Germany, but Monaco is also a sovereign country. And it's not an enclave, it's a homonym.