Impossible to transalte, because of : " number of format specifications in 'msgid' and 'msgstr' does not match ". Transl. from Eng. to Polish.

What was the original text, and what was your proposed translation?

If there is a %d or %s (or similar) part in the original text, then you must have it also in your translation.
Such value is replaced by a given value at run time.

e.g. "Top %d spending" will be (depending on settings) at run time be expanded to "Top 5 spending" or "Top 10 spending"
A German translation might be something like "Die %d größten Ausgaben".
And for Polish you need something similar.

Just a guess, maybe it is "Największe %d wydatki" in Polish (based on some automatic translators on the web).

Heh :) OK, it works, but it is still a bit unclear.
Translation of "Top spending" is easy: "Największe wydatki" alternatively (meaning): "Główne wydatki". But
The only problem is "%d" - it means nothing (in Polish, of course).
Anyway - I entered this translation (Największe %d wydatki) and it was accepted, so now it is possible to guess what it is about. I don't think, that is a problem deserving of a big attention.

Of course there is no meaning of "%d" as such (in no language). Strings like %d and %s are placeholders for values that are filled it at run time.

Another example:
For displaying a message like "You have selected 3 out of 10 items" (with the values 3 and 10 depending on the current situation when using the program) there will be the message string "You have selected %d out of %d items", and at run time the placeholders (%d and %d) will be replaced by the real values (3 and 10 in my example).
The placeholders need to stay untranslated.

If "Największe 5 wydatki" is a correct translation for "the largest 5 expenditure values", and if "Największe 10 wydatki" is a correct translation for "the largest 10 expenditure values", then this translation most probably is correct.

Yes, it is almost correct. I appreciate this short lesson, thanks!
To be specific - almost correct, because of Polish grammatical complication.
"The largest 10 expenditure values" in a proper translation: "10 największych wydatków" ("największe wydatki" - it is nominative, should be: "największych wydatków" - genitive, "10" should be at the beginning). I describe it only to show an example, how my language is bizarre. A little ;)

In that case, you perhaps have to set the translation to

%d największych wydatków