[libcalamares] Add ${LANG} to process expansions

src/libcalamares/utils/CommandList.cpp

@@ -14,6 +14,7 @@
 #include "JobQueue.h"
 
 #include "compat/Variant.h"
+#include "locale/Global.h"
 #include "utils/Logger.h"
 #include "utils/StringExpander.h"
 #include "utils/System.h"
@@ -77,6 +78,16 @@ get_gs_expander( System::RunLocation location )
         expander.insert( QStringLiteral( "USER" ), gs->value( "username" ).toString() );
     }
 
+    if ( gs )
+    {
+        const auto key = QStringLiteral( "LANG" );
+        const QString lang = Calamares::Locale::readGS( *gs, key );
+        if ( !lang.isEmpty() )
+        {
+            expander.insert( key, lang );
+        }
+    }
+
     return expander;
 }
 

src/modules/shellprocess/shellprocess.conf

@@ -12,6 +12,9 @@
 #    system from the point of view of the command (when run in the target
 #    system, e.g. when *dontChroot* is false, that will be `/`).
 #  - `USER` is replaced by the username, set on the user page.
+#  - `LANG` is replaced by the language chosen for the user-interface
+#    of Calamares, set on the welcome page. This may not reflect the
+#    chosen system language from the locale page.
 #
 # Variables are written as `${var}`, e.g. `${ROOT}`.
 # Write `$$` to get a shell-escaped `\$` in the shell command.