Design tab i18n: remove _L override, route strings through gettext

Mirror of snaporca-cad 0f53a09. DesignPanel.cpp pinned every _L() to
wxString::FromUTF8 via a TU-local #define, hardcoding English and bypassing
the gettext catalog — a hard review-blocker for upstream (OrcaSlicer ships
~20 locales). Remove the override so the 463 _L("...") call-sites route
through the real Slic3r::GUI::I18N::translate (wxGetTranslation). All args
are string literals; untranslated strings fall back to the source msgid, so
English is byte-identical while other locales now translate when .po entries
exist. xgettext scans source text, so .pot extraction is unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignPanel.cpp

@@ -40,15 +40,6 @@
 #include "slic3r/GUI/MainFrame.hpp"
 #include "slic3r/GUI/GUI_ObjectList.hpp"
 
-// Apple-esque consistency: the Design tab speaks ONE uniform vocabulary — English — so it
-// never reads as a half-Italian/half-English patchwork against the host's localized chrome.
-// Bypass the gettext catalog for every _L() in this TU (the surrounding Orca UI still follows
-// the app locale; only our tool/card/toolbar strings are pinned). One lever, whole file.
-#ifdef _L
-#undef _L
-#endif
-#define _L(s) wxString::FromUTF8(s)
-
 namespace Slic3r { namespace GUI {
 
 // Format a value with the international ('.') decimal separator regardless of the