> ef menn bjóða fé eða taka fé til liðs sér

"if men offer bribes or take bribes in return for their support/help"

> Í þessum dómi skulu vera allir hinir styrkjustu eiðar og fylgja

tveir menn hverjum eiði er það skulu leggja undir þegnskap sinn er
hinir sverja.

"In this court are to be sworn [by witnesses] only the most binding of
oaths, and two men will back each oath (i.e. swear to the probity of
the witnesses), [two men] who will assert on their honour that which
those others (namely the witnesses) swear."

'leggja e-ð undir þegnskap sinn'
"to asser something on one's honour"
(See Zoega 'leggja' (14) "with preps.")

'fylgja' "follow, accompany" and by extention "back, help" (compare
the English formula "so help me God") [
http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0179.png ].

The first 'er' = the two men. The second 'er' refers back to 'það' =
what 'hinir', the aforementioned persons (the witnesses), swear.