Spanish term (edited): Calidad en que declara - como tal.

There are of course a number of ways to express this. Here is one that would reflect the rather formal language typical of such documents.

I would hope that no one would suggest “acting/declaring as the father himself” as a viable option here.

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Note added at 11 mins (2018-07-27 16:45:05 GMT)
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i.e., “acting” in the *proceeding* resulting in the drafting of the birth certificate

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Note added at 12 mins (2018-07-27 16:46:32 GMT)
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“Appearing” might also work here instead of “acting.”

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Note added at 51 mins (2018-07-27 17:25:56 GMT)
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Or (to stick closer to the original, and reflect the relationship between “declarante” and “declara” in the original):

Acting as informant in this selfsame capacity.