Use TYPO3/CMS/FrontendEditing/Utils/TranslatorLoader to customize or extend language translation on client side. There is a configure function that can be used to merge customize configuration with the default one.

If the configuration is already present, the merge strategy can be used to force an 'merge' or 'override'.

var configuration = {
    translationLabels: {},
    namespaceMapping: {},
var configuration = "none";
TranslatorLoader.configure(configuration, mergeStrategy);

The following example shows how to use it on the server side:

$this->pageRenderer->loadRequireJsModule('TYPO3/CMS/FrontendEditing/Utils/TranslatorLoader', "function(TranslatorLoader) {
        translationLabels: ${translationLabelMap},
        namespaceMapping: ${translationLabelMapping},

No merge strategy is used in the frontend editing default bootstrap. So if it was configured before, no server side translationLabels get configured. Instead it uses the default fallback implementation.

Be aware that the namespaceMapping configuration property may change without deprecation warning since it is not stable.