+static enum protocol_version discover_version(struct packet_reader *reader)
+{

...

+
+       /* Maybe process capabilities here, at least for v2 */
+       switch (version) {
+       case protocol_v1:
+               /* Read the peeked version line */
+               packet_reader_read(reader);
+               break;
+       case protocol_v0:
+               break;
+       case protocol_unknown_version:
+               die("unknown protocol version: '%s'\n", reader->line);

The following patches introduce more of the switch(version) cases.
And there it actually is a
    BUG("protocol version unknown? should have been set in discover_version")
but here it is a mere
  die (_("The server uses a different protocol version than we can
speak: %s\n"),
      reader->line);
so I would think here it is reasonable to add _(translation).