The other steps don't do append. They talk about what fmt contains (e.g. "prefix fmt with...")

The change we should be making should actually be earlier. Something like:

2. For each expression exp in pattern:
   i. Let fmt be the formatted string representation of the resolved value of exp to be appended to the formatted output.

Linkify expressions.

This is weird? Where does B come from? Why is it a "function"? The text above this talks about "bidirectional isolation strategies". I think this needs to be different.

Perhaps:

The Default Bidi Strategy is defined as follows:

1. Let out be an empty string to contain the formatted output of the message
2. Let msgdir...
3. For each expression exp in the pattern:
   1. If exp is a plain literal (text), append exp to out.
   2. If exp is an placeholder, let fmt be the formatted string representation of
      the resolved value of exp.
   3. Let dir be the directionality of fmt...
      // etc.
   4. Append fmt to out.
4. Emit out as the formatted output of the message.

Note that we don't have a name for the non-placeholder bits of text in a pattern. I call them expressions here, but they aren't really expressions.

It would be even better if the section on formatting just above here defined how to make the string output. The "bidi strategy" then could be reference in that set of instructions ("apply the appropriate bidirectional isolation strategy to the contents of expression")

I rewrote this imperatively instead of as a function. I also did a rewrite to call the components of a pattern a "part", and distinguish between text parts and placeholder parts.