Speaking the same language than the person you want to communicate with, already opens doors. When studying languages, you become aware of cultural differences between your native language and the target language. Yet there are obstacles. You just do not study every challenge there will be, when you use a second, or third language in the future. A common situation, when dealing with native speakers of the German language is, that they want to talk to the English-speaking person in English to practice it; it is statistically more unlikely that the native English-speaker speaks German. Germans study English at school. Mainly they do not make the extra commitment to study the "between-the-lines" meanings of sentences.
One big misunderstanding is, that all German-speakers are the same. It is like telling a Scot, that he is no different to a Texan. There are 8 countries, in which German is spoken. Yes, eight! Besides the obvious Germany, there is the German speaking part of Switzerland, Austria (the majority speaks German there), Liechtenstein, Luxemburg, one part in France, called Elsass, the northern part of Italy, called Südtirol, or Alto-Adige, which was Austrian once and Namibia, a former German colony in Afrika. Even in the jungle of South-America you find a valley, where former immigrants kept their Tyrolean (Austrian) dialect.
Most, of course, are Germans. They use there language in a very clear and direct way. The gap in pronunciation between spoken and written language is rather small. Just like the correlation between the written and spoken language already suggests, there is not much difference between the uttered sentence and the meaning of it. Examples are:
Please call me any time. You are invited to live in my house any time.
What the German understands is:
I can call any time. I am invited to live in their house any time.
If they have answered you positively, it is likely, that they really call, or appear!
They just do not understand, that you only say that to be polite. They REALLY feel invited! They have a very different understanding for politeness. Actually it is very rude for them NOT to be invited, although someone had said so. So, just do not use that sentence, if you do not mean it! Simply say good bye, if you want to say good bye.
Yea, I think so too.
The German does not know, that they are supposed to shut up and end the conversation. They would continue to talk to you, as they simply understand, that you think so too. The best way to stop the converstation would be to stop the conversation. You might ad the statement, that you don't want to talk about it any more. They do not necessarily feel offended.
We keep in touch
This is something so common in the English language (in this case almost all of the Engl. speaking parts), that Germans (now referring to all of the German speaking countries), who speak English very well would know, that the actual meaning is, that the communication, the contact has ended so far. But, again: It is still odd to the German eye / ear and therefore better to simply omit the sentence, if you do not want to receive any more messages from that person.
I wish you all the best
This is often used to say that your contact definitely has ended at that point. If it is used that way, something before might have given the idea, that this statement just can't be taken by its word anyway. So the German feels pranked and might react, as he or she is used to react: in a very direct way.
Do not hesitate to say, what you really think, or feel. They are used to that and are not unforgiving.
These are just a few examples, but they probably have given you an idea about the fact, that communicating in a direct and clear way makes the conversartion with a German easier. While the Anglo-Saxon feels offended, when an issue is adressed directly, the Geman speaker can't find a correlation between politeness and using totally different words for an utterance. They do not get the idea, that guessing the meaning of something somewhere between the lines is actually regarded as politeness.
The Swiss-German on the other hand, can cope with that type of conversation much easier. They use that too. They would also smile at you, telling you how fine everything is to complain when you are gone. That explains the issues they have with the Germans. Their directness is not welcome by them. They interpret it as arrogant. A lot of Austrians and German-Italians have that very same issue, although they are somewhere in between the Germans and Swiss, when it comes to using language in a very direct way.
It can be extremely hurtful to find out about these different ways of using language. On the other hand, it is also interesting and listening carefully, as well as asking about the intention of a sentence, can be very interesting.
Making friends with Germans, Austirans, Swiss, ... is not easy; but on the other hand: If they call you a friend, they stay one forever, unless they find out, that you haven't taken that relationship seriously.