Introduce LanguageTagIcu::getDisplayName()

To be used in generating display names for on-the-fly language
tags in the language listbox.

Change-Id: Ibb36fe312e550f77f1e2d3acc2dbff9e83c15f7b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/120797
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

i18nlangtag/source/languagetag/languagetagicu.cxx

@@ -46,4 +46,26 @@ icu::Locale LanguageTagIcu::getIcuLocale( const LanguageTag & rLanguageTag, std:
            );
 }
 
+// static
+OUString LanguageTagIcu::getDisplayName( const LanguageTag & rLanguageTag, const LanguageTag & rDisplayLanguage )
+{
+    // This will be initialized by the first call; as the UI language doesn't
+    // change the tag mostly stays the same, unless someone overrides it for a
+    // call here, and thus obtaining the UI icu::Locale has to be done only
+    // once.
+    static thread_local LanguageTag aUITag( LANGUAGE_SYSTEM);
+    static thread_local icu::Locale aUILocale;
+
+    if (aUITag != rDisplayLanguage)
+    {
+        aUITag = rDisplayLanguage;
+        aUILocale = getIcuLocale( rDisplayLanguage);
+    }
+
+    icu::Locale aLocale( getIcuLocale( rLanguageTag));
+    icu::UnicodeString aResult;
+    aLocale.getDisplayName( aUILocale, aResult);
+    return OUString( reinterpret_cast<const sal_Unicode*>(aResult.getBuffer()), aResult.length());
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

include/i18nlangtag/languagetagicu.hxx

@@ -15,6 +15,7 @@
 #include <string_view>
 
 #include <i18nlangtag/i18nlangtagdllapi.h>
+#include <rtl/ustring.hxx>
 #include <unicode/locid.h>
 
 class LanguageTag;
@@ -49,6 +50,22 @@ public:
         Always resolves an empty tag to the system locale.
      */
     static  icu::Locale     getIcuLocale( const LanguageTag & rLanguageTag, std::u16string_view rVariant, std::u16string_view rKeywords);
+
+    /** Obtain the display name for a language tag.
+
+        Gets a localized name from icu::Locale::getDisplayName(). Meant to be
+        used only with language tags that do not have a language list entry in
+        svtools/inc/langtab.hrc STR_ARR_SVT_LANGUAGE_TABLE, thus used by
+        SvtLanguageTable::GetLanguageString()
+
+        @param  rLanguageTag
+                The language tag for which the name is to be displayed.
+
+        @param  rDisplayLanguage
+                The language in which the name is to be displayed.
+                Usually Application::GetSettings().GetUILanguageTag().
+     */
+    static  OUString        getDisplayName( const LanguageTag & rLanguageTag, const LanguageTag & rDisplayLanguage );
 };
 
 #endif  // INCLUDED_I18NLANGTAG_LANGUAGETAGICU_HXX