When you donate used clothing to certain charities, they sell it to brokers, who in turn pack it up and ship it to various African countries.
There, brokers distribute the used clothing, at large markups, throughout the country. That’s why you might see a Zambian boy wearing a Bart Simpson T-Shirt, or an Iraqi man wearing a T-shirt for the 1994 Kansas State Representative race of Ed Pugh, or a Nigerian post office employee wearing a baseball cap that says "forget about the love and passion, just whack it up her doggy fashion" (same site).
Does this also happen to German clothes, I wonder? It’s probably harder to tell, because Germans are much less likely than Americans to wear clothes printed with a raunchy/ephemeral message. Africans might be wearing cast-off German clothes, but it would be much harder to immediately identify the clothes as specifically German. Or maybe I’m wrong, and there are Liberian dock workers proudly sporting Marius Müller–Westernhagen concert T-shirts.
Now for the socially redeeming part of this post (you knew it was coming). A documentary film has been made about used Western clothes in Africa, and their baleful impacts on local manufacturers:
What happened to all of the Zambian clothing manufacturers? Mark O’Donnell, spokesperson for Zambian Manufacturers, explains that in 1991, when the country’s markets were opened to free trade, container load after container load of used clothing began to arrive in Zambia, undercutting the cost of the domestic manufacturers and putting them out of business. The skills, the infrastructure and the capital of an entire industry are now virtually extinct, with not a single clothing manufacturer left in the country today.