A linguist friend once described Bahasa Indonesia, the national language of Indonesia, as “the language killer.” By this he meant that the spread of Indonesian has had a remarkable impact on the thousands of local languages in Indonesia, with little or no official status. One amazing consequence of this is that languages like Javanese, with over a hundred million speakers, may actually be endangered.
The social aspects of Indonesian’s spread are a rich area of study. Here is one illustration: using a .1% sample of the 2010 Indonesian census, I have modeled the probability that an ethnic Javanese respondent speaks Indonesian at home as a function of gender, age and age squared, whether he or she lives in an urban or rural area, and the percentage of the respondent’s district who is ethnic Javanese. Below I plot the predictive margins: estimates of the probability of speaking Indonesian for various interesting combinations of these variables.
The findings are actually a bit reassuring: in heavily Javanese areas (basically, Central and East Java) Javanese is being spoken at home by children. But as Javanese move outside of these regions—and especially in urban areas—the probability of speaking Indonesian jumps dramatically. And the curves by age are predictions of the future: children who speak Indonesian at home will almost certainly not be able to teach their children Javanese.
Still, Indonesian will have a long way to go to kill Javanese, because most Javanese still live primarily among other Javanese and primarily in rural areas, as the chart below illustrates.
So long as this is the case, Javanese will be safe. Unfortunately, the Indonesian census records neither bilingualism nor linguistic competence; if it did, we could learn the extent to which bilingualism in Indonesian is degrading competence in Javanese. That is the frontier for research—how Indonesian kills them softly.
tompepinsky.com 
Charley Sullivan January 29, 2014
Several other variables that have an effect in my experience (though without any large-scale data):
1. inter-marriage: mixed marriage families, where only one parent speaks Javanese, raise their kids almost exclusively in Indonesian.
2. Religious/cultural affiliation: Javanese Christians: the kids speak Javanese, often quite well. NU: the kids speak Javanese, but not refined Javanese; the time that would be spent on that goes into Arabic increasingly. Muhammadiyah and other modernist groups: Bahasa Indonesia rules the house, and since teachers in Muhammadiyah schools come from across the country, there is sometimes little informal usage of Javanese in educational settings.
3. As Javanese flattens, and language levels become less pronounced (an “Indonesianisation” of Javanese?) it becomes easier to use, and more people end up using it . . . even non-Javanese in cities like Yogya or Semarang. Solo, not so much . . .
tompepinsky January 29, 2014
Ha! I also have results for both (1) and (2). Christians: yes, but even here, urban/rural makes a big difference: urban Christian Javanese are very much Indonesian speakers. For mixed marriages, the results are actually not quite so clean. If mom speaks Indonesian but not dad, it’s more important than dad but not mom. (And it gets higher for Javanese over 50, because if you had a parent who spoke Indonesian in 1965, you’re almost certainly of a type that speaks Indonesian.) If both parents speak Indonesian, though, the probability of speaking Indonesian is basically 1.
Jennifer Frentasia January 29, 2014
I think the Chinese Indonesian case gives a very interesting perspective. Aside from places like Medan and Singkawang, many Chinese Indonesians do not speak Chinese dialects, since the law did not allow them to be taught. Yet, they are not as comfortable with Bahasa Indonesia as they would be with the local languages. For example, I would say that I’m most comfortable talking in Suroboyoan.
Also, could the level of education matter too in the function?
tompepinsky January 30, 2014
So fascinating…the reality is that we are really talking about something like “Middle Indonesian,” not Bahasa Indonesia Yang Baik dan Benar. And the results for education are massive…as you might expect, education changes everything.
Nana April 17, 2014
It is interesting. I have Malaysian/Bruneian friends (malays) visiting me several years ago. When i picked them from the airport (Surabaya) they told me that they were amazed by the number of chinese speaking bahasa in the baggage claim. I told them that those are Chinese Indonesians, they speak bahasa just like everybody here speak bahasa, not only that those living in javanese speaking area will speak javanese also with the accent sometimes even heavier than some javanese people. I think they had their jaw dislocated after hearing my explanation that day :p
tompepinsky April 17, 2014
I actually had a similar experience in reverse…after 6 months in Indonesia, I moved to Malaysia. It took me awhile to figure out that most Chinese who I met couldn’t—or wouldn’t—speak Malay. I made a lot of faux pas during those first weeks.