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That must be a very frustrating thing to happen, but I think I can help you with it. (I do comprehend German language some myself, so this is a hopeful situation.)
What model is the Garmin please? Was it originally speaking English?
Does the writing/text on the screen still read in English?
OK. I am looking up how to switch language on it. I expect to post again with results within 10 or 15 minutes or so. Thank you for your patience meanwhile.
Admittedly not, payment is expected upon delivery of a good answer. If you wish to abandon the question, or cancel it, please let me know.
On the Garmin, from the home screen or main screen, tap on
Settings (which is a wrench icon and at the lower right corner of the screen.) In German, that's "Einstellungen".
Then tap on
Language (which is an icon that looks like lips, and is in the bottom center of the screen.) In Germans, that's "Sprache".
And change that to
(Aenglische or Englisch, in German.)
And approve that change.
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