We don't implemented configurable subject and message text, because Joomla! has a pretty good system on board.
You can use the Joomla! language overrides. How to use those you can read in the official Joomla! documentation for Language Overrides in Joomla
There you can search for example for:
- ; %1$s name, %2$s -> SW_KBIRTHDAY_DAY(S), %3$s -> SW_KBIRTHDAY_ADD_AGE, %4$s -> SW_KBIRTHDAY_DATE
Keep in mind to use the placeholders in the correct meaning. You can use those in any order in your string as long they written exactly the same way.