Commonness of casual phrases like "あたし" and "ちっちゃい" I came across a Japanese girl in Germany who talked using words like instead of , or instead of . When I asked her, she said that these words or usages of words are more common in Japan. * Is this true? * How does the majority of Japanese people judge this? * Shall learners pay attention to such details or avoid this topic entirely?

Atashi is for females. Chicchai can be seen as more emphatic "tiny". They are both quite common. Remember that in most textbooks for any language the most ideal form of the language (often judged so by prescriptive grammarians) is taught. As you familiarize yourself with real world usage (through travel, friends, and media) you will discover all sorts of words you were never taught because they seemed "too casual". DO pay attention because casual speech is appropriate for casual situations; nothing sets you apart as a gaijin more than using polite speech in every single conversation you have.