Notice how your item has a weird display name, such as item.tutorial.my_item? This is because your item's name doesn't have a translation in your game's selected language. Translations are used to support multiple different languages for a single string.

You can use lang files to provide translations for translatable strings in-game. You'll need to create a file with an appropriate file name for your language– to find your languages' code, visit Minecraft Wiki. English is en_us. Once you have your language code, create a JSON file at resources/assets/modid/lang/; a full example for an English translation file would be resources/assets/tutorial/lang/en_us.json.

After you've created the lang file, you can use this basic template to add translations:

resources/assets/tutorial/lang/en_us.json

{
  "item.tutorial.my_item": "My Item",
  "item.tutorial.my_awesome.item": "My Awesome Item",
  [...]
}

where the keys in JSON is the translation key. If you're following along in the wiki tutorial, remember to change namespace tutorial to the namespace you actually use.

Whenever a function accepts Text, you have the option of giving it a new LiteralTextContent() or Text.literal() (for versions since 1.19), which means minecraft will use the string in the constructor argument as-is. However, this is not advisable because that would make it difficult to translate that text to another language, should you wish to do that. This is why whenever a Text object is needed, you should give it a new TranslatableTextContent() or Text.translatable with a translation key, and then translate the key in the lang file.

For example, when adding a tooltip, do:

@Override
public void appendTooltip(ItemStack stack, TooltipContext context, List<Text> tooltip, TooltipType type) {
    // 1.18.2 and before
    tooltip.add(new TranslatableTextContent("item.tutorial.fabric_item.tooltip"));
 
    // 1.19 and later
    tooltip.add(Text.translatable("item.tutorial.fabric_item.tooltip"));
}

And then add in the lang file:

resources/assets/tutorial/lang/en_us.json

{
  "item.tutorial.fabric_item.tooltip": "My Tooltip"
}

And the tooltip will be displayed as “My Tooltip”!

Say you want the text to change based on some variable, like the current day and month. For a dynamic number, we put a %s where you want the number to show in the lang entry value, for example:

resources/assets/tutorial/lang/en_us.json

{
  "item.tutorial.fabric_item.tooltip": "My Tooltip in day %s, and month %s" 
}

Then we pass the variables we use in our string by the order it appears in the text. First the day, then the month:

int currentDay = 4;
int currentMonth = 7;
 
// 1.18.2 and before:
tooltip.add(new TranslatableTextContent("item.tutorial.fabric_item.tooltip", currentDay, currentMonth));
 
// 1.19 and later:
tooltip.add(Text.translatable("item.tutorial.fabric_item.tooltip", currentDay, currentMonth));

In translations, the order of variables may change. Therefore, you can explicitly specify the index of variables by using something like %1$s and %2$s.

If you want for it to literally show %, use %%.

For more information, see Java String.format (it works the same way).

The line-break \n work finely in command outputs. However in many cases, such as item tooltips, they may not workd correctly. So in order to have a string with multiple lines in the tooltip, you must split the translation key into multiple keys:

resources/assets/tutorial/lang/en_us.json

{
  "item.tutorial.fabric_item.tooltip_1": "Line 1 of my tooltip", 
  "item.tutorial.fabric_item.tooltip_2": "Line 2 of my tooltip" 
}

Then add the TranslatableTextContent parts individually:

// 1.18.2 and below:
tooltip.add(new TranslatableTextContent("item.tutorial.fabric_item.tooltip_1"));
tooltip.add(new TranslatableTextContent("item.tutorial.fabric_item.tooltip_2"));
 
// 1.19 and later
tooltip.add(Text.translatable("item.tutorial.fabric_item.tooltip_1"));
tooltip.add(Text.translatable("item.tutorial.fabric_item.tooltip_2"));

Line 1 of my tooltip
Line 2 of my tooltip