private String buildCacheKeyString(String sourceLanguage, String targetLanguage) {
        return sourceLanguage + ":" + targetLanguage;
    }
Build the String to be used as the key into this CachedTranslator's cache.

@param sourceLanguage The source language of translation.
@param targetLanguage The target language of translation.
@return The string to be used as the key into this CachedTranslator's cache.
tika-translate/src/main/java/org/apache/tika/language/translate/impl/CachedTranslator.java