Namibia gained its independence in 1990, but South Africa was had the apartheid policy until 1994.
How did Namibia (which was South African land until 1990) deal with this after independence? Was racial segregation immediately criminalized, or did apartheid continue in Namibia as well?
They arguably dealt with it before independence, in the Constitution promulgated in February 1990 - see Article 23, page 15, which is explicit. Although SWAPO (the liberation organization, South West African Peoples' Organisation) did not get a 2/3 majority and could not dictate the constitution, they found remarkable readiness to compromise and broad agreement on the need to end racial measures--even though the realization of that ambition has not exactly been perfect, of course.
The independence of Namibia resulted from negotiations between Cuba, Angola, and SA, with mediation by the superpowers, which recognized the UN's revocation of South Africa's trusteeship in 1966. The Acts ending Apartheid in SA came after this was negotiated, so unlike the extension of those Acts after 1951, those did not apply, nor did they need to, given the election of Sam Nujoma and the independence of March 1990. Marion Wallace's History of Namibia (Oxford: OUP, 2011) covers the essentials in chapter 10.