Updates a PO message with a new translation.

For plural messages the PO file itself defines how many plural forms the language uses (one msgstr[n] per form), so only the forms tracked by the translation entry (msgstr[0] and msgstr[1]) are replaced. Higher forms (msgstr[2]+) already present in the file are preserved as-is; they are only filled with the generic plural translation when still empty, so newly translated messages don't end up with blank forms.

An explicit :plural_translation_2 key on the translation entry overrides msgstr[2] when present and non-empty.