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/locale/testtranslations.fr.po

@@ -0,0 +1,9 @@
+# Dummy entry. This is a test to make sure that a parser error is not thrown
+# if the following line is msgctxt.
+msgctxt "testtranslations"
+msgid "Dummy entry"
+msgstr "Dummy result"
+
+# Used for translating the mod title
+msgid "Test translations"
+msgstr "Test translations (French)"

games/devtest/mods/testtranslations/locale/translation_po.fr.po

@@ -0,0 +1,22 @@
+# Test Plural-Forms parsing
+msgid ""
+msgstr ""
+"Plural-Forms: nplurals=2; plural= (n-1+1)<=1 ? n||1?0:1 : 1?(!!2):2;"
+
+msgctxt "testtranslations"
+msgid "Testing .po files: untranslated"
+msgstr "Testing .po files: translated"
+
+msgctxt "testtranslations"
+msgid "@1: .po singular"
+msgid_plural "@1: .po plural"
+msgstr[0] "@1: .po 0 and 1 (French singular)"
+msgstr[1] "@1: .po >1 (French plural)"
+
+#, foo bar fuzzy
+msgctxt "testtranslations"
+msgid "Testing fuzzy .po entry: untranslated (expected)"
+msgstr "Testing fuzzy .po entry: translated (wrong)"
+
+msgid "Testing .po without context: untranslated"
+msgstr "Testing .po without context: translated"

games/devtest/mods/testtranslations/locale/translation_tr.fr.tr

@@ -0,0 +1,2 @@
+# textdomain: testtranslations
+Testing .tr files: untranslated=Testing .tr files: translated