It's probably not Germanic, it may go back to a Celtic word for 'oak, oak-bark'
(tann, though it's only recorded in Breton and Cornish);
Medieval Latin tannare 'to tan', and noun tannum, may be from that Celtic word ,
and Old English (ge)tannian could be from Latin or Celtic;
there's Old French tanner, with cognates in Italian, Spanish, Portuguese,
but the only Germanic language other than English that has the word seems to be (Middle) Dutch.
Interestingly (well, to geeks like me it is), modern Celtic languages don't seem to have an equivalent,
they use phrases meaning something like 'treat leather'.
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