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>

games/devtest/mods/testtranslations/test_locale/readme.txt

@@ -0,0 +1,4 @@
+The translation files in this directory intentionally include errors (which
+would be reported when someone starts the devtest game in the de locale). This
+allows the unittest to check that the translation file reader also handles
+files that include errors.

games/devtest/mods/testtranslations/test_locale/translation_po.de.po

@@ -0,0 +1,42 @@
+# This file is used by the C++ unittest for testing the parser
+msgid ""
+msgstr "\n\n\n"
+"Plural-Forms: nplurals=2; plural=n!=1;"
+"\n\n\n"
+
+msgid  "foo"
+  msgstr "bar"  
+
+msgid "Untranslated"
+msgstr ""
+
+#, fuzzy
+msgid "Fuzzy entry"
+msgstr "Wrong"
+
+msgid "Multi\\""line\n"
+"string"
+msgstr "Multi\\\""  "li\\ne\nresult"
+
+msgctxt "Something" in "between"
+msgctxt "String does not end
+msgstr "Lost string"
+msgid "Wrong order"
+
+msgid "Singular form"
+msgid_plural "Plural form"
+msgstr[0] "Singular result"
+msgstr[1] "Plural result"
+
+msgid "Not enough value"
+msgid_plural "Not enough values"
+msgstr[0] "Result"
+
+msgid "Partial translation"
+msgid_plural "Partial translations"
+msgstr[0] "Partially translated"
+msgstr[1] ""
+
+msgctxt "context"
+msgid "With context"
+msgstr "Has context"