Unfortunately, it seems that Endnote does not have the present capacity for accommodating multiple in-text citation formats based on reference type as you described in your posting.
How many Polish and foreign sources are you citing and what output style are you using? The numbers may suggest modifying your output style’s citation template to use either the “p.^pp.” abbreviation or the “s.^ss.” abbreviation?. Without knowing the answer let’s just say there are more foreign sources. A possible workaround in this case is to:
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Let the current citation format of your output style could do to use the “p.^pp.” abbreviation for foreign sources. However, when you need to insert an in-text citation for a Polish source, insert the citation then edit the citation to include the “, ss.23-89” in the field of the dialog box (refer to attached image #1 for example of editing the citation in the result).
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Then within each of the Polish and foreign sources records in insert either the pp. or ss. abbreviation with the corresponding numbers in either the Pages field (or create a custom field). Refer to image #2. Then add the Pages field (or custom field) to the footnote or bibliography template. This will cause the correct abbreviation to appear based on the reference itself.
