Why does my tongue itch/burn when I eat pineapple and kiwi?

Pineappleitis. Pineapple commonly causes minor skin irritation because it's highly acidic. It's common for your tongue to feel gritty, itchy and mildly painful after eating pineapple. If you develop severe pain, itching, burning or swelling in your tongue from eating pineapple, you should talk with your doctor.
Enzymes & Acids. Some fruits, such as pineapples, have proteolytic enzymes that break down proteins. Additionally, they have a higher citric acid & malic acid content - which can cause burning sensations, irritation, redness, & "itchiness". If you absolutely love pineapples & kiwis, you can always pause between portions & rinse your mouth, which may lessen discomfort. Possible allergy if swelling if so avoid then.