[mbielski1337]

Adds Polish as an interface language.

What's in it

• New crates/ui-egui/src/i18n/pl.rs with 970 entries. That's every key in the Japanese reference catalog plus all the extra labels the other tables translate (dialogs, tooltips, the PivotTable pane, Analysis ToolPak, Solver, spelling, Format Cells). I took the union of keys from all the existing language files so nothing in Polish falls back to English where another language already has a translation.
• Language::Pl in i18n.rs: name "Polski", code pl, system tags pl-PL / pl_PL.UTF-8 resolve to it, so a Mac set to Polish starts in Polish.
• Tests in the existing style: the catalog completeness check now covers Polish, plus a few tr() and from_tag() asserts.

Formulas stay on the canonical en-US locale, like every interface language except Spanish.

Translation notes

• Written from the meaning of the English labels. Feature names use the vocabulary Polish spreadsheet users already know (skoroszyt, arkusz, formuła, Scal i wyśrodkuj, Zablokuj okienka). Longer sentences are my own wording.
• Two-line ribbon labels keep a \n at a natural break, same as the Czech table.
• Relative ages use abbreviations ("{n} tyg. temu", "{n} mies. temu", "{n} l. temu") because Polish plurals change with the number (2 lata vs 5 lat) and the pattern only gets one form.
• Placeholders ({n}, {title}, {count}...) and shortcut hints (⌘X, ⇧F9) are kept as they are.
• "Outline" is a single key used both for the border preset and the PivotTable report layout. I went with "Kontur" (border); happy to change if the layout use matters more.

Overlap with #650

I saw #650 also adds Polish (about 440 entries). This one covers the full 970, including the dialog and tooltip strings. Where both translate the same label the wording is nearly identical. I'm fine with whatever works best for you: merge this one, or merge #650 first and I'll rebase this into a follow-up that adds only the missing labels.

[mbielski1337]

I'm a native Polish speaker and went through the wording myself.
I haven't built it locally, so I'm relying on CI here. I checked by script that every key matches an existing key exactly, there are no duplicates, and every placeholder from the English key appears in the translation.