The I18nTextReader could be improved to be able to escape certain characters, either with a replacement table or with a preconfigured mode.
That would allow us to avoid having one message for JS, one for XML, with different escapings (backslashes before quotes and double quotes in JS), and more generally to avoid thinking about escaping when writing internationalized messages.

However, some messages are currently already escaped for JS, so a full pass over existing catalogues will be required.