this line works fine and if not found inside DB it creates new one.

GetDictionaryItemByKey 

next line GetDictionaryItemChildren throws Exception:

System.InvalidOperationException : Current has not been initialized on Umbraco.Core.Persistence.Mappers.MappingResolver. You must initialize Current before trying to read it.

here ist the whole test class

[TestFixture] 
public class TestImport : BaseUmbracoApplicationTest
        {
            [Test]
            public void TestImportDictionary()
            {
                var dictContainer = ApplicationContext.Current.Services.LocalizationService.GetDictionaryItemByKey("Abteilungen.Intranet") ??
                                         ApplicationContext.Current.Services.LocalizationService.CreateDictionaryItemWithIdentity("Abteilungen.Intranet", null);


                var children = ApplicationContext.Services.LocalizationService.GetDictionaryItemChildren(dictContainer.Key);
            }
        }