The phrase is saying that the document is:
For the purposes of true extract from the Population Information System in Finland
A true extract is a legalese term used to mean an extract that has been certified in some fashion. For example, the Australia Taxation Administration Act contains:
(2) Where, pursuant to a State tax law, a copy is made of a document, a State taxation officer may certify the copy to be a true copy.
(3) Where a document is obtained pursuant to a State tax law, a State taxation officer may certify an extract taken from the document to be a true extract.
Because the document you have is called "extract from the population system", the headline is saying that it is created to be a certified extract. It could also be saying that the undersigned authority is "certifying this true extract from the..."
Another example of its use is A true extract out of the Commons journal of the most principal proceedings of that honourable House, in this last short meeting; in order to the preservation of the King and kingdom from the growth of popery, and also for reducing the growing greatness of France.
However, as FumbleFingers points out the phrase "for true extract" is probably just a poor translation. A quick Google search of the phrase yields dictionaries for translation rather than other instances of the phrase in common usage. So "true extract" is fine, but the phrase "for true extract" is likely a bad translation of a longer phrase.