OET interlinear YHN (JHN) 5:31

uW Translation Notes:

Note 1 topic: figures-of-speech / explicit

ἐὰν ἐγὼ μαρτυρῶ περὶ ἐμαυτοῦ, ἡ μαρτυρία μου οὐκ ἔστιν ἀληθής.

if I /may_be/_testifying concerning myself the testimony ˱of˲_me not is true

Here Jesus is referring to a rule in the law of Moses. According to Deuteronomy 19:15, a statement had to be confirmed by at least two witnesses in order to be considered true in legal decisions. If your audience is not familiar with the law of Moses in the Old Testament, then you can state this explicitly. Alternate translation: [You know that the law of Moses states that if I testify about myself, my testimony is not true]

Note 2 topic: figures-of-speech / explicit

ἐὰν ἐγὼ μαρτυρῶ περὶ ἐμαυτοῦ

if I /may_be/_testifying concerning myself

Jesus assumed that his listeners understood that he was referring to testifying about himself without any other witnesses. If this might confuse your readers, you could express the meaning explicitly. Alternate translation: [If I testify about myself without any other witnesses]