Germany has a rich and varied history and, as a result, uses different phrases and figures of speech in different areas.  "Moin Moin!" or just "Moin!" is another way to say "Hello!" in Northern Germany, Hamburg, East Frisia and nearby areas. It is considered an all-day greeting for everyone. "Grüß Gott" translates into "May God greet you," and is considered a way to say hello to everyone in southern Germany, Bavaria or in Austria. It is a formal phrase to greet a stranger or a teacher. "Servus!" is another greeting you'll only hear in the south of Germany or Austria, translating to "hello."

TL;DR in Vietnamese:
Hãy quen dần với tiếng địa phương.