In the sentences the numbers are usually written in English, eg 100. If you were speaking that, would it be one hundred or hyaku? Would 2 be two or ni?
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It would be in Japanese if you were to speak it. These numerals have become almost universal, but they would always be spoken in whatever language it’s used in.
It would also be to your benefit to read them in the native language too, when you come across them. Just to help you grow accustomed to numbers in that language.
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Yahalo 
@veryslowlearner
Like @conan says, you are supposed to read numbers in the language of the text.
In Japanese, Hindu-Arabic numeral system is used when the text is written horizontally.
If the text is vertical, Japanese numerals are used instead.
Though sometimes you can see Japanese numerals with horizontal text.
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The complicated answer is this: it depends on the context and counter.
In most cases you’ll use the Japanese number pronunciations, however, there are a few contexts in which numbers would be pronounced with a Katakana syllable pronunciation of american pronunciation of numbers. This will be few and far between though.
More important though is that the pronunciation of the numbers can change depending on the counter used (especially the numbers 1 through 10).
For example, these 3 counters all have different pronunciations of the number 1,
1人 <- ひと・り
1台 <- いち・だい
1分 <- いっ・ぷん
The first used a Kunyomi reading of 1, the second uses a On’yomi reading of 1, and the third uses a special pronunciation to make it easier to say quickly.
Further the pronunciation of the counter can also change with the number. For example,
1人 <- ひと・り
2人 <- ふた・り
3人 <- さん・にん
or
1分 <- いっ・ぷん
2分 <- に・ふん
3分 <- さん・ぷん
You’ll have to learn each on a case by case basis. This can be a pain at time but there are only a few that you’ll need on a regular basis.
For an in depth dive see this article:
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Cheers for the detailed info.
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