1:27a

Some translations, including Berean Standard Bible, New International Version, Good News Translation, New Living Translation (2004), and Contemporary English Version, begin a new sentence here. But the Greek text continues the sentence begun in 1:26 (see the New Revised Standard Version). You may do so also if it is natural in your language. For examples of other ways to divide the sentences, see the General Comment on 1:26–27.

He is the One who comes after me: Jesus was coming after John in the sense that his ministry would begin after John’s. John had already begun his ministry, but Jesus had not yet begun his ministry. So consider what verb tense is most natural in your language. For example:

Even though I came first (Contemporary English Version)
-or-
I ⌊preached⌋ before him, he will ⌊preach⌋ next.

1:27b

the straps of whose sandals I am not worthy to untie: In NT times people normally wore sandals on their feet. These sandals were fastened with thongs (leather strings or strips/straps). Slaves normally untied the thongs of the sandals of their masters. John was saying that he was not worthy (did not deserve) to serve Jesus as a slave. In some languages you may need to make this explicit. For example:

I am not even worthy to be his slave. (New Living Translation (2004))