It’s not ideal, but this code should work for now (only from the Gateway/Perspective):
from org.apache.commons.lang3 import LocaleUtils
from com.inductiveautomation.ignition.gateway import IgnitionGateway
from com.inductiveautomation.ignition.common.i18n.translation import TranslationPackageDiff
translationLocale = LocaleUtils.toLocale("en_US")
modifications = TranslationPackageDiff()
modifications.addTranslation("term", translationLocale, "translation")
IgnitionGateway.get().getLocalizationManager().applyDiff(modifications)