Handling of community-provided translations

For preparation fix use of language and translate service: The language changed event only needs to be used once per lazy loaded i18n module.

Language selection in HeaderComponent has been refactored and improved. Furthermore LanguageService has been simplified and extended.

For this add new Language model which is using new LanguageCategory enum. With this the language category can be detected for extra handling when selecting or prevent from choosing automatically.

After a community language has been selected, an info dialog opens. For this the YesNoDialogComponent has been modified to be used as simple info dialog and is now named BaseDialogComponent.

Furthermore spanish has been added as first community-provided translation.

Also highlight selected language in menu and stop translating language name for better UX.

At least add tests for language service.

Closes #1091 (closed)