Gettext and plural support for client-side translations (#14726)

---------

Co-authored-by: Ekdohibs <nathanael.courant@laposte.net>
Co-authored-by: y5nw <y5nw@protonmail.com>
Co-authored-by: rubenwardy <rw@rubenwardy.com>

src/gettext_plural_form.h

@@ -0,0 +1,33 @@
+// Minetest
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#pragma once
+#include <string_view>
+#include <memory>
+
+// Note that this only implements a subset of C expressions. See:
+// https://git.savannah.gnu.org/gitweb/?p=gettext.git;a=blob;f=gettext-runtime/intl/plural.y
+class GettextPluralForm
+{
+public:
+	using NumT = unsigned long;
+	using Ptr = std::shared_ptr<GettextPluralForm>;
+
+	size_t size() const
+	{
+		return nplurals;
+	};
+	virtual NumT operator()(const NumT) const = 0;
+	virtual operator bool() const
+	{
+		return size() > 0;
+	}
+	virtual ~GettextPluralForm() {};
+
+	static GettextPluralForm::Ptr parse(const size_t nplurals, const std::wstring_view &str);
+	static GettextPluralForm::Ptr parseHeaderLine(const std::wstring_view &str);
+protected:
+	GettextPluralForm(size_t nplurals): nplurals(nplurals) {};
+private:
+	const size_t nplurals;
+};