First pass at language plugins for internationalisation

Still quite a few details to work out, but this shows the basic idea of
re-using the theme mechanism to handle language plugins.

Comments and questions welcome.

languages/en-US/languageprompt.tid

@@ -0,0 +1,3 @@
+title: $:/lingo/ControlPanel/Language/prompt
+
+Hiya! Current language:

languages/en-US/plugin.info

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+{
+	"title": "$:/languages/en-US",
+	"name": "en-US",
+	"plugin-type": "language",
+	"description": "English (US)",
+	"author": "JeremyRuston",
+	"core-version": ">=5.0.0",
+	"dependents": ["$:/languages/en-GB"]
+}