These cookies have no certification on the box itself, it is made in a spanish speaking country. yet they have a sticker with a kosher certification put on them.
is this reliable?
This is not a practical question.
This label is on many products in Israel. In my experience, most people do trust the sticker labels; however, to be safe, when you see a product with the kosher certification stuck on a sticker to the box, it certainly cannot hurt to call the certifying agency to confirm that they do supervise that particular item.
As a separate issue, many people do not rely on the particular kosher certifying agency shown on this label (Triangle-K). The majority of major kashrus agencies do not recommend consuming food with only the Triangle-K certification.
See this question and its answers.