It is possible to use localized labels directly in Custom components.
Examples
For a static message in the HTML section:
<ft-localized-label context="$CONTEXT_ID" key="$KEY_ID"></ft-localized-label>
For a dynamic message in the JS section:
const context = "$CONTEXT_ID";
const label = "$KEY_ID";
await FluidTopicsCustomI18nService.prepareContext(context);
const message = FluidTopicsCustomI18nService.resolveMessage(context, label);
document.querySelector(".class-example").innerText = message;
Replace $KEY_ID and $CONTEXT_ID with the correct information from the custom-$LanguageCode-messages.xml file.