Hello and thanks for your question.
If I've understood you correctly, you want to know how to fill in these gaps:-
I, __________________, am proficient in _________________ and I can confirm that I have translated this document to the best of my ability and they have confirmed to me that the contents are accurate.
Here's what you need to say:
I, (your full name)
of (your address)
am proficient in (the language that your friend speaks) (if there's a good reason add it here, for example -
because it is my mother tongue, or,
because I lived for x
number of years in (name of country)
or, because I have studied (name of language)
to degree level and I can confirm that I have translated this document to (name of your friend)
to the best of my ability and he or she has confirmed to me he or she has understood the contents, ( then if the statement is a statement by your friend, you can add)
and that that the contents are accurate.
Dated: .......(the date when you sign).
I hope this helps and I wish you the best of luck.
Thanks and best wishes...