Add Romanian (ro_RO) localization

Romanian was absent from this fork and from upstream, so there was no
catalog to reuse. Machine-translated with a local qwen3.6 model against the
union of both forks' .pot files; 99.0% coverage (5472 singular + 9 plural).
Header credits it as machine translation pending native review.

Registering wxLANGUAGE_ROMANIAN in supported_languages[] is what actually
exposes the language: the Preferences dropdown filters the installed
dictionaries against that allowlist, so a valid .mo alone would ship a
language nobody could select.

Translations that failed to preserve their printf/boost placeholders were
left empty rather than shipped, since msgfmt --check-format is fatal in
run_gettext.sh and would break the build on all three platforms. Those 57
strings fall back to English. Verified: run_gettext.sh exits 0 and the
compiled catalog has 0 placeholder mismatches.

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

src/slic3r/GUI/Preferences.cpp

@@ -153,7 +153,8 @@ wxBoxSizer *PreferencesDialog::create_item_language_combobox(wxString title, wxS
         wxLANGUAGE_PORTUGUESE_BRAZILIAN,
         wxLANGUAGE_LITHUANIAN,
         wxLANGUAGE_VIETNAMESE,
-        wxLANGUAGE_THAI
+        wxLANGUAGE_THAI,
+        wxLANGUAGE_ROMANIAN
     };
 
     auto translations = wxTranslations::Get()->GetAvailableTranslations(SLIC3R_APP_KEY);