what is the english equivalent of the above german proverb?
and how would you say it in french?

My dictionary says there's a literal translation: "Cobbler, stick to your last." and it also gives "Every man to his trade."

I've seen "cobbler, stick to your last" in lists of proverbs but I don't think I've ever heard anyone use it. I'm not sure how many people would even know what a last is. I can't think of any other equivalent to the German, though.
I've heard it as "shoemaker, stick to your last." I agree it's rather archaic.
Supposedly, it originated in the Latin "Ne sutor supra crepidam." World Wide Words says
>The Latin writer Pliny recorded that Apelles, the famous Greek painter who was a contemporary of Alexander the Great, would put his pictures where the public could see them and then stand out of sight so he could listen to their comments. A shoemaker once faulted the painter for a sandal with one loop too few, which Apelles corrected. The shoemaker, emboldened by this acceptance of his views, then criticised the subject’s leg. To this Apelles is reported as replying (no doubt with expletives deleted) that the shoemaker should not judge beyond his sandals, in other words that critics should only comment on matters they know something about. In modern English, we might say “the cobbler should stick to his last”, a proverb that comes from the same incident. (A last is a shoemaker’s pattern, ultimately from a Germanic root meaning to follow a track, hence footstep.)
>What Pliny actually wrote was ne supra crepidam judicaret, where crepidam is a sandal or the sole of a shoe, but the idea has been expressed in several ways in Latin tags, such as Ne sutor ultra crepidam ( sutor means “cobbler”, a word still known in Scotland in the spelling souter ). The best-known version is the abbreviated tag ultra crepidam, “beyond the sole”
Edited by: nutraxfornerves
Doggone it! The software puts the full quote in italics, so I had to change my italics to underline. And then I couldn't underline wthout sticking spurious spaces before & after parentheses.
From Wer-Weiss-Was. I have put the French phrase in bold
Schuster bleib bei deinem Leisten sagt man, wenn einer über seine Möglichkeiten und Fähigkeiten hinaus will; sprich nicht von Dingen, die du nicht verstehst. Das Sprichwort hat noch bis in den Schlagertext aus den fünfziger Jahren des 20. Jahrhunderts nachgewirkt:
Schuster, bleib bei deinen Leisten,
Schöne Frauen kosten Geld.
Plinius berichtet in seiner 'naturalis historia' (35,10) von Apelles, dem Hofmaler Alexanders des Großen, daß ein Schuster ihn anläßlich einer Ausstellung seiner Bilder getadelt habe, weil der Maler eine Sandale falsch dargestellt hatte. Apelles verbesserte daraufhin das Bild; als der Schuster nun noch weitere Einwände gegen das Gemälde vorbrachte, soll der Maler gesagt haben: »Ne sutor supra crepidam« (Schuster, nicht weiter als die Sandale). Unser Sprichwort ist also keine direkte Übersetzung dieser Pliniusstelle. Man mag es später jedoch darauf bezogen haben. Vgl. französisch 'Mêle-toi de tes oignons': Kümmere dich um deine eigenen Zwiebeln
[Lexikon der sprichwörtlichen Redensarten: Leisten, S. 3. Digitale Bibliothek Band 42: Lexikon der sprichwörtlichen Redensarten, S. 3781 (vgl. Röhrich-LdspR Bd. 3, S. 956) (c) Verlag Herder]