private SignificanceModel handleFallBackToUnknownLanguage() throws IllegalArgumentException {
        var unknownModel = significanceModelRegistry.getModel(Language.UNKNOWN);
        var englishModel = significanceModelRegistry.getModel(Language.ENGLISH);

        if (unknownModel.isEmpty() && englishModel.isEmpty()) {
            throw new IllegalArgumentException("No significance model available for unknown or english language");
        }

        return unknownModel.orElseGet(englishModel::get);
    }
Sets significance values on word items in the query tree.

@author Marius Arhaug
container-search/src/main/java/com/yahoo/search/significance/SignificanceSearcher.java