Maybe a combination: a small icon with the three flags in one row, and underneath, the text "German post".
this does not address the main problem that flags do not correspond to languages. German is spoken in more than "the three" countries, and in these countries there are more languages than German. Thus, using flags is insensitive and alienating to German speakers from other countries (e.g., Belgium), and non-German speakers from these three countries (e.g., Lugano).
And this is just for German! The situation is even more complicated for nearly all other languages in the world. Please, never use flags of states to identify languages. I have been on the receiving end of this unnecessarily offensive thing, and it did not feel nice.
The alternative is to use ISO (En, De, Es) in brackets before the text begins. i.e. [DE] Wie Studien verdreht werden
ISO language abbreviations would be a better alternative than the flags but I don't think anything is needed at all; anyone who knows German will recognize the post titles written in German and anyone who doesn't know it will still recognize that they're not in English.
The links do have `hreflang="de"` which in theory can programmatically indicate the language used at the destination, I'm not sure there's any technology that takes advantage of that. They could use a ::before pseudo-element with content: attributes(hreflang) to display the ISO code instead of the flag images. I would like to see `lang="de"` attributes on the German headings and paragraphs on the home page, they would tell screen readers to switch to a voice appropriate for the language when read aloud.
Do you think so?
Which flag would you use for euros? Obviously not the european union flag, since there are many countries inside the EU with a different currency (like Sweden or Poland) and even countries outside the EU whose currency is the euro (Montenegro, Kosovo, Andorra, ...).
I think the fact is that languages simply don't have visual symbols for them. They are spoken or written, not "looked at". Languages are linear, not 2-dimensional. Euros and Dollars have their symbol because that's what is printed on the 2D surface of bills.
If the context is currencies, everyone would know that a european union flag refers to the euro.
it's a visual cue, not a translation into hieroglyphics.
In the case of your website, however, I don't see why you need to specify the language of each entry. Is it a sort of "warning" for non-english entries? It seems unnecessary. And why aren't entries in English marked with the flag of the USA? (or Canada, or South Africa).
If you need a corresponding URI fragment, the ISO language codes exist for that reason.
Just from the title I can immediately see that "Die Metapher der Stadt für das Bauen und Betreiben von Softwareprodukten" is definitely not going to be in English.
I assume readers fluent in German don't need to be told that post is in German, the same way I don't need a little UK or US flag to know "March 2018 Book Recommendations" is in English.
If you can read the title and introduction you will immediately know which language its written in with no need for flags at all.