Service to implement the retrieval of language services.
Inheritance: LanguageServiceBase, ILanguageService
            public void ReturnsStringForMvvmAssembly()
            {
                var languageService = new LanguageService();

                Assert.AreEqual("Warning", languageService.GetString("WarningTitle"));
                Assert.AreEqual("Warning", languageService.GetString("WarningTitle"));
            }
            public void ReturnsNullForNonExistingResource()
            {
                var languageService = new LanguageService();

                Assert.AreEqual(null, languageService.GetString("NonExistingResourceName"));
            }
            public void ThrowsArgumentExceptionForNullResourceName()
            {
                var languageService = new LanguageService();

                ExceptionTester.CallMethodAndExpectException<ArgumentException>(() => languageService.GetString(null));
            }
            public void ThrowsArgumentExceptionForNullLanguageSource()
            {
                var languageService = new LanguageService();

                ExceptionTester.CallMethodAndExpectException<ArgumentNullException>(() => languageService.RegisterLanguageSource(null));
            }
            public void ReturnsStringForCoreAssembly()
            {
                var languageService = new LanguageService();

                Assert.AreEqual("{0} has the following warnings:", languageService.GetString("WarningsFound"));
            }