I am recently working in a project. There I need to convert language from English to Japanese by button click event. The text is in a div. Like this:
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If you mean translating the text, you cannot do it easily. To get some idea of the best attempts that software can make at translating natural languages, go to Google Translate or Babelfish. It's not that good, but it's sometimes an intelligible starting point.
If you just mean setting the language attribute on an element, then assign a new language code to the lang property of the div element object.
document.getElementById("normaltext").lang = "en-US";
I don't know the language code for Japanese; possibly ja-ja.