Question: Is it wajib (compulsory) for the immigrants in Europe and America (and other similar countries) to strive for teaching their children Arabic, and that ignorance of Arabic may lead in the future to ignorance of the main Islamic body of knowledge, and that will naturally lead to less familiarity with religious teachings and loss of faith?
To teach them Arabic is wajib only to the extent which is necessary for performing their religious duties that have to be done in Arabic (e.g., recitation of the Opening chapter of the Qur'an, a second chapter, and other wajib recitations in salat). Teaching more than that is not wajib as long as it is possible to provide them with religious knowledge in a foreign language.
Of course, it is recommended to teach them the holy Qur'an [in Arabic]; rather it is important to teach them Arabic in a precise form so that they may benefit from the basic sources of Islamic teachings, especially, and foremost among them, after the holy Qur'an, is the Prophetic sunna and the sayings of the Ahlul Bayt (extended family of the Prophet, i.e. the Shia Imams).