Fix regression in wxTranslations::AddCatalog()

Do load the catalog corresponding to the language of "msgid" strings in
the source code, only skip the languages strictly less preferred than
it.

This avoids incompatibilities with pre-3.1.2 behaviour and avoids
breaking existing applications relying on the old behaviour.

Closes https://github.com/wxWidgets/wxWidgets/pull/1081

Closes #18297.

docs/changes.txt

@@ -108,6 +108,10 @@ Changes in behaviour which may result in build errors
 3.1.3: (released 2019-??-??)
 ----------------------------
 
+All:
+
+- Fix regression in wxTranslations::AddCatalog() in 3.1.2 (Tomasz Słodkowicz).
+
 All (GUI):
 
 - Fix wxInfoBar close button size in high DPI (Stefan Ziegler).