So how would I localise an item's name in code, as opposed to a language file?

Don't know why you'd want that but override Item.getItemStackDisplayName

Thanks! What I want to do is have a few localisations such as metal.tin and metal.copper, and match them with component.plate and component.powder, so I can combine them to make Tin Plate, Tin Powder, Copper Plate, Copper Powder, etc.

Mixing and matching like that isnt advised because other languages have different syntaxes so its not always {Material} {Form}

What Lex says is very true, but don't let that discourage you - translations can be very flexible with just a little bit of formatting, as they ultimately use the Java Formatter class:

// Lang file:
yourmod.item.name=%s %s // results in "Tin Plate"
yourmod.metal.tin.name=Tin
yourmod.component.plate.name=Plate

// Item code:
@Override
public String getItemStackDisplayName(ItemStack stack) {
// Make sure to use #translateToLocalFormatted and pass the translations for each component as an argument
return StatCollector.translateToLocalFormatted(getUnlocalizedName() + ".name",
  StatCollector.translateToLocal("yourmod.metal.tin.name"),
  StatCollector.translateToLocal("yourmod.component.plate.name"));

Of course, it's up to you to determine which metal and component to use in the translation, probably using some kind of Object retrieved from NBT. But anyway, then languages with different ordering rules can translate it like so:

// taking advantage of Formatter syntax to re-order the arguments:
yourmod.item.name=%2$s %1$s  // results in "Plate Tin"