Please make sure of following steps:
1> You need to place whole po and mo file generated from poedit, [ click here for poedit tutorial ] there inside theme folder:
2> Then be sure you can find you translated string for specific string on po files
3> Be sure you are using file with prefix is de_DE
4> Finally select the language from site language section. Go here:
Go to dashboard -> settings -> Genearal -> Site Language [ Duetch ]
Let us know if those steps help you on this. and also we had provided you the reference links on previous reply, please go through all this.