Since placeables can be of mixed directionality, I'd like to suggest that Fluent's FSI/PDI insertion for string placeholders is added to requirements.

This allows a variable like userName to be inserted in a string with different directionality and inform the layout of the possible direction change.

W3C backlog: https://www.w3.org/International/articles/inline-bidi-markup/
Fluent wiki: https://github.com/projectfluent/fluent/wiki/BiDi-in-Fluent