Bit of a long shot, but I think the answer is:
To this riddle one shall add "meta"
'est' or 'esti' translates as 'this' in some languages and 'mate' is an anagram of 'meta', so depending on whether or not we use the 'one' in the clue we have 'est'+'i'+'mate' or 'esti'+'mate'.
Like a program and an SE site
programme is like program in how it's pronounced and in some definitions but with an extra 'me'. Adding to 'an', we get 'mean' (this part may need more explanation). SE refers to Standard Error and a mean and standard error are part and parcel of an estimate. The word 'site' can also combine with 'meta' to make 'estimate', playing into the theme of a 'meta' riddle.
For the answer you have some data
An estimate needs some input data. The riddle's first two lines also yield words useful for creating the 'estimate' anagram. Also, 'at' is some of data which we can combine with letters yielded from the last line.
Yet not always you will get it right
If you estimate, you don't always get it right. Also, 'not always' could be a synonym for sometimes and the 'right' of sometimes could be 'etimes'. Combining this with the 'at' of line three we can rearrange to again get 'estimate'.