Strings() formats with string.format, which fills directives in argument order. A language that puts two values the other way round from English ("ATTACK de PIKACHU" for "PIKACHU's ATTACK") could not say so: the translation either kept the English order or swapped the values. The official FireRed, Gold and Red localizations reorder values like this in dozens of battle and menu lines.
A translation can now number its directives the way POSIX printf does ("%2$s de\n%1$s monte!"), and Strings() hands the arguments over in that order. Flags and widths are kept ("%3$03d") and "%%" stays a literal percent. A translation numbers all of its directives or none, and may only number arguments the source has; it may leave one out, as the official translations sometimes drop a name. Otherwise it falls back to the English source and logs one warning, like a translation with the wrong number of directives.
Sources never carry numbered directives, and a translation without them takes the same string.format path as before, so English and existing catalogs render the same text. A translation that still cannot format its arguments falls back to the English source formatted with them, instead of the raw template. An older engine given a numbered translation cannot format it and prints the template, so a catalog using numbered directives needs this engine.
thibautbus