Note that testing a translation needs the bundle, not the locale

Fixing the Czech plurals turned up a trap worth recording: String(localized:
locale:) reads the language from the bundle, while locale: only drives
number and date formatting. A test that passes a Czech locale and nothing
else quietly compares English strings — ours failed with "1 coupon" ==
"1 kupón", which reads as broken plurals when the plurals were fine and
the test was looking in the wrong place.

Also that a plural variation must reference the number, so a key
extracted as "%@" cannot carry one and the build says so outright.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -12,7 +12,7 @@
       "name": "pdat-swift-practice",
       "source": "./pdat-swift-practice",
       "description": "Curated Apple platform findings with sources, plus practices learned from production incidents.",
-      "version": "0.5.0",
+      "version": "0.6.0",
       "author": {
         "name": "PDAT s.r.o."
       },

pdat-swift-practice/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "pdat-swift-practice",
-  "version": "0.5.0",
+  "version": "0.6.0",
   "description": "Curated Apple platform findings with sources, plus practices learned from production incidents.",
   "author": {
     "name": "PDAT s.r.o.",

pdat-swift-practice/skills/apple-findings/references/localization.md

@@ -43,6 +43,31 @@ count wrapped in parentheses — "Send unsent receipts (3)" — dodges the
 problem entirely and is a reasonable fallback where a plural form would
 be awkward.
 
+## Testing a translation needs the right bundle, not the right locale
+
+`String(localized:locale:)` looks like it renders a given language. It does
+not: `locale:` drives number and date formatting, while the language comes
+from the **bundle**. A test that passes `locale: Locale(identifier: "cs")`
+and nothing else reads the English strings and reports them as if they
+were Czech.
+
+```swift
+let path = Bundle.main.path(forResource: "cs", ofType: "lproj")!
+let czech = Bundle(path: path)!
+String(localized: "\(1) coupons", bundle: czech, locale: Locale(identifier: "cs"))
+```
+
+Our first attempt at the plural test got this wrong and failed with
+`"1 coupon" == "1 kupón"` — which at a glance reads as "the plurals are
+broken" when in fact the English plurals were working and the test was
+looking in the wrong place.
+
+Also: a plural variation must reference the number. A key extracted as
+`%@ coupons` cannot carry one, and the build fails outright with "Plural
+variation requires referencing the number in the string" — usable
+feedback, and a reason to keep counts typed as `Int` so the key comes out
+as `%lld`.
+
 **Step forward?** Yes, and overdue. This is the kind of defect that never
 shows up in a test and that a Czech speaker notices immediately.