I wonder if this would be true for Native languages

[Rudy Troike]

The most successful Native-English dual language program I know of was a truly
bilingual Navajo-English program at Rock Point, Arizona. It was well-researched
and had a strong academic curriculum, in which students outperformed those in
purely ESL programs, and became highly literate in both Navajo and English,
proving IT CAN BE DONE. I have met some of the graduates of the program here
at the University of Arizona, and found them very impressive. Nothing has been
said about the program here on ILAT, and I don't know if it is still continuing.

Conversely, as I have mentioned before, a UNM dissertation some years ago showed
that Navajo children who were placed in English-only Head Start
programs in order for them to learn English before starting school, were
found to perform LESS WELL by the 3rd grade than comparable students who
began school monolingual in Navajo.

The study also showed that the early exposure to English had degraded
children's grammatical competence in Navajo, suggesting a cognitive
connection between loss of native language competence and lower achievement
in English-only schooling.

  --Rudy