This can mean that 1) someone found that person in the act, "is discovered in an act of sin," or 2) that person committed the sin without intending to do evil, "gave in and sinned."
These words treat the Galatians as though they are all one person to emphasize that he is talking to each of them. AT: "yourselves … you(plural)" (see UDB) or "I say to each one of you, 'Look out for yourself ….'"
"for fear that you should be tempted to sin too" or "in order that you should not be tempted to sin." If you need to use active voice, it would be best not to mention "Satan" or "the devil," since they are not mentioned anywhere in this letter. AT: "so that the tempter does not tempt you." (See: [[rc://en/ta/man/translate/figs-activepassive]])