They might be fast-growing; the immigrant communities ae in another country, where they don't know the local language and can feel isolated, so when a Jehovah's Witness knocks the door, is polite, even somewhat friendly, it's a very positive experience - the person acceps a study just because the person is welcoming, in a foreign land.
The reality is, most of the persons who join these foreign language groups do it because they are bored with English ministry, so go and try some foreign foods, learn a few phrases, BUT few really STUDY the language. They read prayers translated by interested persons, have sisters translate their talks, etc. and just hope people answered the Watchtower questions right, as they have no idea what was said. Many of these foreign language meetings are conducted in a mix of English and another language - so much for Holy Spirit giving appointed ones skills to complete the tasks, like when the temple was built. This idea that you no longer hear what is being said is not true.
There is a lot of racism within the organisation, or more opposition to languages, i.e. you can be black, but you will speak English - many elders are negative towards foreign language groups, and many in those groups are only fond of them as they don't want to mix with the English. These groups and congregations can be very political, though yes, there are a few that are thought well of, especially if the CO gets positive about it, and such a group will look good, help one progress to be an elder, etc..