Sayyiduna Umar (may Allah be pleased with him) said that when the tongue is cut off there is the full diyah for it; if the tongue is damaged to the extent that speech is stopped, there is also the diyah for it; and whatever is less than that, the diyah will be in proportion to it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16253