Germany owned quite a bit of Africa and even had a fairly big port in the Chinese area of Qingdao. They owned massive chunks of Africa like Cameroon and Tanzania and half of Papua New Guinea. I know that the Germans had to give up a lot (all?) of their colonial assets after their loss in the First World War. Even with that though, the German peoples still would've had time to firmly implant the German language in those areas. How come this never stuck?
In the case of Tanzania, Swahili was chosen by German Missionaries as the lingua franca to be taught to German East-Africa . The Germans controlled German South-East Africa for a much shorter time than say, the french in West Africa/Mali/Algeria .For example, The Colony of German South-East Africa only existed from 1891 to 1919, whereas Senegal had french trading posts in 1624. The comparison in particular isn't perfect, but you can see that having the properties removed after WWI certainly reduced their cultural impact.
This question is really complex because Cochinchina and French Indochina were both under French rule for a long time, yet French as a language never caught on like it did in Morocco, and Lebanon.
Other reasons I think french may have caught on are that being such a large colonial bloc in West Africa and to a lesser extent Madigascar and the Indian Ocean that it may have made sense to keep using the language in the post-colonial period . Also, there was sort of an "economy of scale" in that the language was used across French West Africa .
In addition, French wasn't tied to an ethnic group, the way other native languages that acted to facilitate trade were (like hausa) .
This sort of is a poor comparison to Swahili because there was an ethnic group that spoke the language, but it is at least worth mentioning. In the case of Vietnam, they didn't need a semi neutral language the way French acted in certain west-african countries .
https://www.africa.upenn.edu/K-12/French_16178.html
http://m.voanews.com/a/francophone-west-Africa-English-language/1545406.html
For reference, Rwanada has recently started emphasizing English in lieu of French.
German has a stronger presence in Namibia than in Tanzania, but it really is a hard question to answer.
In relation to Quingdao, even Macau has few Portuguese speakers these days, so it seems the small mass of language speakers made it fade away pretty quicky as a second language. Even the Japanese rule in Korea didn't create the long lasting use of the language seen in Morocco and West Africa .
This paper outlines the situation in Tanzania a bit more: http://ojs.ub.gu.se/ojs/index.php/modernasprak/article/download/1187/1026
http://www.afrykanistyka.uw.edu.pl/pliki/files/publikacje/legere.pdf
In relation to west africa: http://www.degruyter.com/view/j/ijsl.2000.issue-141/ijsl.2000.141.75/ijsl.2000.141.75.xml
I suppose the TLDR would be that the germans were busy with genocide in German South West Africa, while they just accepted Swahili as an easier and more prolific language of administration in German South East Africa .