Define a new environment variable to determine the directory where
message catalogs ('.mo' files) are to be be looked up.

LibreOffice already has environment variables such as 'BRAND_BASE_DIR'
and 'BRAND_SHARE_RESOURCE_SUBDIR' but these are not helpful in this
case.

Patch by Ludovic Courtès <ludo@gnu.org>.

--- libreoffice-6.1.5.2/unotools/source/i18n/resmgr.cxx	2019-03-10 16:22:21.352963608 +0100
+++ libreoffice-6.1.5.2/unotools/source/i18n/resmgr.cxx	2019-03-10 16:30:20.090848346 +0100
@@ -144,6 +144,11 @@ namespace Translate
         rtl::Bootstrap::expandMacros(uri);
         OUString path;
         osl::File::getSystemPathFromFileURL(uri, path);
+
+        const char *cLocaleDir = ::getenv("LO_LOCALE_DIRECTORY");
+        if (cLocaleDir != NULL)
+            path = OUString::createFromAscii(cLocaleDir);
+
         OString sPath(OUStringToOString(path, osl_getThreadTextEncoding()));
         gen.add_messages_path(sPath.getStr());
 #if defined UNX && !defined MACOSX && !defined IOS && !defined ANDROID