Bob's idea is excellent (if you planned ahead).
Setting the language is a character-based setting. If you decided to make the entire document bold (horrors!) or change the type-size or change the typeface, those would also be character-based.
Many people base styles on other styles instead of making them stand-alone, which makes it easier to add in this kind of change.
For example:
Body copy stands alone
Bullets are based on body copy
First para is based on body copy
When body copy is edited, all three change.
This far along and with 200 or 300 styles--if they are not in separate frames--you could Select All with the Type tool and change the language. It is not a best practice to do local formatting, but you could then redefine the styles if you had time.
Note that changing the language does not translate. It spell checks and hyphenates in the appropriate language.
If you want a "mother of all styles", you need to create it yourself and base all styles on it. There is one base style built into InDesign, but most people don't use it and create their own instead. You could spot check some of your styles and see what they are based on. If they have the same parent (unlikely, but possible), you can redefine the parent and the children will change. It's worth looking.