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In page Treaty of Trianon:

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The census of 1910 classified the residents of the Kingdom of Hungary by their native languages[1] and religions, so it presents the preferred language of the individual, which may or may not correspond to the individual's ethnic identity. To make the situation even more complex, in the multilingual kingdom there were territories with ethnically mixed populations where people spoke two or even three languages natively. For example, in the territory what is today Slovakia (then part of Upper Hungary) 18% of the Slovaks, 33% of the Hungarians and 65% of the Germans were bilingual. In addition, 21% of the Germans spoke both Slovak and Hungarian beside German.[2] These reasons are ground for debate about the accuracy of the census.