Oh, we would easily understand you. You would learn High German, and we all understand that. It's just that most people do not actually speak it. There are just a few areas where they actually speak something very close to High German, and these areas are areas where the local dialect has simply died out. In contrast to other countries, where the dialect of the court or of the capital city became the standard, in Germany it was a conglomerate of southern dialects based on the Lutheran Bible; and Luther used words and grammar from a number of dialects. The north, which had different dialects (modern English is a Northern German dialect by way of some Danish and a lot of French), practically had to learn a new language. That's why northern dialects almost died out. For example, I live just above the demarcation line of north and south as far as dialects are concerned. Our local dialect is ailing, only very few people actually speak it. I can't even understand it myself, though I can read it.
But as a foreigner, the High German you learn will be understood and people will try to speak in what they think is High German, too. You'll get by. After all, High German is used in the media, so everyone who has a TV set or a radio in Germany knows the language.![]()