1) Assimilate to Slavic customs 2) To make Russian 
• (v. t.) To make Russian, or more or less like the Russians; as, to Russianize the Poles.
Rus'sian·ize transitive verb To make Russian, or more or less like the Russians; as, to Russianize the Poles. 
to make Russian; impart Russian characteristics to. · to subordinate and force to adhere to Russian culture, ideology, etc. · (sometimes l.c.) to treat (leather) by a process similar to that used on Russia leather or by any process that will produce such leather.