The population of Latvia today is estimated at slightly below the census population of 1935, and falling (WP, citing the Latvian statistical bureau). I believe these figures reflect a consistent geography and the boundaries have not substantially changed since 1920, but I haven't been able to precisely confirm that.
Many Baltic and Eastern European states show a similar pattern, but none quite match your 1950 cut-off. Estonia and Lithuania have had a similar but less pronounced population decline, and the last time they had their current populations was probably circa 1965-70. Hungary's population now is comparable to its population in the mid-1950s. In the Balkans, Bosnia & Herzegovina and Serbia have populations now comparable to their populations in the late 1960s. All of these are still falling gradually.