Assigning a language to text determines which spelling and hyphenation dictionary is used. Assigning a language does not change the actual text.
To apply the language only to selected text, select the text.
To change the default dictionary used in InDesign, choose the language with no documents open.
To change the default dictionary for a specific document, choose Edit > Deselect All, and then choose the language.
InDesign uses Proximity language dictionaries for both spelling and hyphenation. These dictionaries let you specify a different language for as little as a single character of text. Each dictionary contains hundreds of thousands of words with standard syllable breaks. Changing the default language does not affect existing text frames or documents.
You can customize language dictionaries to ensure that any unique vocabulary you use is recognized and treated correctly.