As a matter of fact, Chinese people have lived in the Indonesian archipelago since at least the 13th century. However, their population grew rapidly during the colonial period due to workers were contracted from their home provinces in southern China.
The total population of Chinese Indonesian itself is estimated to be around 2,832,510 in 2010 census, and nearly half of the population inhabits Sumatra island. The reason is mainly because it’s the ideal location to arrive from the mainland, China.
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Now, let us progress to answer the main question about the proportion of Chinese Indonesians who cannot speak Indonesian.
Actually, it is hard to guess the exact proportion, since there has been no official research about it yet. Even so, we can still make some assumptions or even a reasonable guess based from many relatable sources and statistics taken.
However, before that we should talk about what factors caused Chinese Indonesians to not be able to speak Indonesian in the first place.
Education
The main factor is of course education. Many Chinese Indonesians who could not speak Indonesian are mostly those who either wasn’t schooled in Indonesia or has no formal education at all.
Back then a lot of Chinese migrants, or often called as the old first-generation, did not achieve proper education. This is probably due to the discrimination against them and getting worse to the resentment of ethnic Chinese economic aptitude grew in the 1950s as native Indonesian merchants felt they could not remain competitive.
Another possible reason behind this is because of the individuals themselves, these old first-generation probably chose to not learn Indonesian.
As for the Second generation Indonesian-Chinese living in Indonesia, they can at least speak a basic Indonesian language or a dialect. Next to the following generation, they are already fully aware of Indonesian because most of them has been educated using the language.
You can check How to teach Bahasa Indonesia to Preschoolers – Guidance and Tips for further information.
Living Environment
In other cases, living environment can become one of the factors that affect these people understanding of Indonesian language.
Even though it is very unlikely that Chinese Indonesian cannot speak Indonesian nowadays unless they grow up abroad, there are still some who are not be able to speak Indonesian, as the effect of this living environment factor.
For example, there are people who mostly live in area where there are a lot of people who speak non-Indonesian language. It can be in rural area where the inhabitant speak the regional language daily to the point that they rarely speak Indonesian.
Occupation
This case is usually a house-wife who never given chance to socialize and has lived in Indonesia prior to independence (thus couldn’t attend Indonesian school). They are usually the second generation whose parents are migrants from China and still struggling to live in Indonesia.
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Understanding those factors is essential as we are going to try guessing the percentage of Chinese Indonesian living in Indonesia who cannot speak Indonesian.
Based from Indonesia Demographics Profile 2018, around 8.58% of Indonesian population age between 55-64 years and over 65 years is around 7.01%.
Taken those numbers are the population of the first and second generation of Chinese Indonesian and the total population itself is 2,832,510, then we can assume there is around or less than 400.000 Chinese Indonesian that are unable to speak Indonesian.