Diccionario InDesign

Brief tour on a Language setting: it's a character-level attribute. Can be set in a Paragraph style, and overridden by setting in a Character style, which in turn can be overridden manually. To clean up all this, a script might help.

Here is a simple javascript one-liner to apply Spanish language to the whole document, in InDesign with English UI:

app.activeDocument.stories.everyItem().appliedLanguage="Spanish";

if you have tables in your doc, add this line, too:

app.activeDocument.stories.everyItem().tables.everyItem().cells.everyItem().texts.everyItem().appliedLanguage="Spanish";

Be aware, that settings in your existing Para and Char styles remain untouched, just overridden. That's not a good practice at all, however, Hyphenation and Check Spelling will work correctly now (these are two cases when Language setting really matters).

If you want to do it in a really right way, that said, update all your styles to a new setting, you should use a bit more sophisticated solution.

Here's a direct link to free LFCorullón's script that does exactly what you need:

Just install and run it, select desired language from the list, click OK, and you're done.