Well, I would say if it's done legally, and if the immigrants intend to learn the language, assimilate to the host country's culture, governing principles, etc., then it is not an invasion.
On the other hand, if the immigrants have no intention of assimilating, but on the contrary work to establish, say (to cite a random example), a theocracy whose tenets are inimical to the culture and governing principles of the host country, then yes, it IS an invasion, and countries that permit this to happen are committing suicide.