阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。
It's estimated that up to 7,000 different languages are spoken around the world. About 90% of (they) are used by less than 100,000 people.
Languages are grouped into families that have shared origins. For example, English is related German and Dutch, and they are all part of the Indo-European family of languages.
About 2,200 of the world's languages can (find) in Asia, while Europe has merely 260. Nearly every language uses a similar grammatical structure, even though they may not be linked in vocabulary or origin. Communities which are (normal) isolated from each other may have developed multiple languages.
The ease or difficulty of (learn) another language can depend on your mother tongue. general, the closer the second language is to the learner's native tongue and culture in terms of vocabulary, sounds or sentence structure, the (easy) acquisition (习得) will be.
Dutch is said to be the easiest language for native English speakers (pick) up, while research shows that for those native English speakers already know another language, some of the most difficult languages to get your head around (be) Arabic, Mandarin Chinese, Japanese and Korean.