en_tn/jhn/08/18.md

I am he who testifies about myself

Jesus testifies about himself. Alternate translation: "I give evidence to you about myself"

the Father who sent me testifies about me

The Father also testifies about Jesus. You could make it explicit that this means Jesus' testimony is true. Alternate translation: "my Father who sent me also brings evidence about me. So you should believe that what we tell you is true" (See: rc://en/ta/man/translate/figs-explicit)

the Father

This is an important title for God. If your language must state whose Father this is, you could say "my Father" since Jesus switches to that in the following verses. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)