For all who don't see my comment, you can easily modify the language in a document. Just save it in .idml and use the following regex:
Search: (?<=AppliedLanguage\="\$ID/)\w+(: )*?\w+(?=")
If you want to:
1/ modify the language option of all the para styles in the document,
2/ delete all the language options manually used in the document,
using the "French" language.
… But t o do this, You'll need to go to the dark side of the Force!
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