It is not fair to say that the DA has nationalization on its agenda
As for nationalism it is a little more nuanced and complex with nationalism being an ideology and sentiment rather than a policy and in terms of supporting - or failing to oppose - ANC policies of a nationalization nature because of certain nationalist sentiments one could probably quite fairly muster a charge: for example if we regard BEE as a form of nationalization* of economic decision making rights then the DA's support albeit qualified and and and ... could be interpreted as such; similarly the DAs policy on mineral wealth and spectrum does include premises that are tantamount to the state acquiring an nationalized interest. Some libertarians would argue that to support tax is support nationalization - and they have a point.
* this is an arguendo statement, I am not arguing the case that it is but the argument can be made if one treats control of a body as property rights which some legal scholars argue they should be or are.