Well there is a simple answer to that… Because we still say their word for themselves, in a similar way to the modern english word “us”. Their word is deutch. it comes from their word for “us”. In holland they say dutch, which is the SAME word… a cognate of us and deutch. The country of Germany isn’t germane to the language.
The name “German” comes from the words the romans (and hence the latin language) used to name a Gaul tribe name , and the area on the map, and hence … the country which came into being in 1871… (but did exist as the Holy Roman Empire - an alliance with many areas such as Bavaria , and the feudal Bavarian Circle , Prussia, and so on,which is a long complicated history with definitions and redefinitions ) However, the
Deutch and the other “Germanic” languages are in fact from Scandanavia.
Paul the Deacon wrote in the Historia Langobardorum that the Lombards descended
from a small tribe called the Winnili who dwelt in southern Scandinavia…
Note that the Lombards then went on to rule (what is now) Italy till 774 right ?
Well thats how Old Germanic got to the HRE (Deutchland) … migration.
Its not as simple as Mr Lombard went south and imposed Germanic language there… there were numerous migrations southward… An earlier invasion was the Teutons going up the Rhine, and then circling around France and the Iberians before defeat. They were probably a mix of rebels, eg Gauls and Celts with some Teutons, anyway.
Of course there were many more, with so many less dramatic they are obscure…
But Norse migrated south.
And so the modern english word “Germany” is not a good example… its an academic word locked in by its origin in the Latin language.
A good example for finding out how “Rosetta stones” help tell us about historic pronunciations is the Icelandic, as it is extremely INSULAR. That means that the changes in pronunciation are evolutionary inside Iceland, and not imposed by invasion (mostly.)
See History of Icelandic - Wikipedia
Fixed rules of spelling didnt exist. They wrote down words phonetically. If the pronounciation changed, the spelling changed. So by tracking many words spelling through the scandanavian languages, the pronunciation can be followed back from modern pronounciation to then… The probably of accuracy is increased when there are many modern pronunciations and spellings, and the tracebacks arrive at the same pronunciation…