The problem I see with that, is that you are making the assumption that the word for days should always come after the number etc. By using a substitution in the translation, you give more control to the translator to decide the best way to display in their language.
There are also cases where I need to append a character after the translation, and it just feels like it’s getting messy when I have to start using some spans to handle that, in addition to the translator problems.
But, either way, I think we’ve got a good starting point for a recommended approach to translating an options page, and people can always build off it as needed.