Add new class LangStringSet
LangStringSet is defined now as: LangStringSet = Dict[str, str]
It's reasonable to create a new class LangStringSet where it's possible to choose e.g. default language to show in repr. A new class would also be usefull for type checking: now it's allowed to set any param to Dict[str, str] where LangStringSet required and it will not be detected.