Implement platform_get_locale_date_format for Linux

Note that this currently, this isn't implemented in the C++ standard
library. This means that date_order() will always return no_order,
causing this function to always return the default
DATE_FORMAT_DAY_MONTH_YEAR, as is the case before this commit too.

Once date_order() will be properly implemented in the standard library
someday, it will however start returning the right order for the current
locale, causing this function to start working as intended.

distribution/changelog.txt

@@ -9,6 +9,7 @@
 - Feature: [#7868] Placing scenery while holding shift now scales appropriately with zoom levels.
 - Fix: [#3177] Wrong keys displayed in shortcut menu.
 - Fix: [#4039] No sprite font glyph for German opening quotation mark.
+- Fix: [#5548] platform_get_locale_date_format is not implemented for Linux.
 - Fix: [#7204] Object source filters do not work for RCT1, AA and LL.
 - Fix: [#7440] Memory leak. All system memory used.
 - Fix: [#7462] Guest window goes beyond the map edge on a spiral slide.

src/openrct2/platform/Posix.cpp

@@ -27,6 +27,7 @@
 
 #    include <libgen.h>
 #    include <locale.h>
+#    include <locale>
 #    include <pwd.h>
 #    include <stdlib.h>
 #    include <sys/file.h>
@@ -421,7 +422,22 @@ uint8_t platform_get_locale_temperature_format()
 
 uint8_t platform_get_locale_date_format()
 {
-    return DATE_FORMAT_DAY_MONTH_YEAR;
+    const std::time_base::dateorder dateorder = std::use_facet<std::time_get<char>>(std::locale()).date_order();
+
+    switch (dateorder)
+    {
+        case std::time_base::mdy:
+            return DATE_FORMAT_MONTH_DAY_YEAR;
+
+        case std::time_base::ymd:
+            return DATE_FORMAT_YEAR_MONTH_DAY;
+
+        case std::time_base::ydm:
+            return DATE_FORMAT_YEAR_DAY_MONTH;
+
+        default:
+            return DATE_FORMAT_DAY_MONTH_YEAR;
+    }
 }
 
 datetime64 platform_get_datetime_now_utc()