Are discount rules working only in default language ?
Are you using WPML ?
Here we go!! Let's find out the steps to get it sorted.

By default, Woo discount Rules is compatible with WPML. 

But, In certain discount scenarios, you will have to create one rule per language... (By selecting those language specific categories or products) 

Step 1:
Choose the secondary/relevant language from Wordpress dashboard.

Step 2:

Navigate to Woocommerce ---> Woo discount Rules.
Create the same discount rule for that specific language.
Make sure to select the products/categories for the chosen language. 

Example:

Let's take an example scenario on how to create separate discount rules for different language.

1. Provide 10% off on Bag for English language product

2. Provide 10% off on Bag for French product

Scenario 1:

Provide 10% off on Bag for English language product

Rule setup:

Cart page:

Scenario 2:

Provide 10% off on Bag for French product

Rule setup:

Cart page:

Likewise, we need to create separate rules per language for categories, tags, attributes and so on...

Also, by creating separate rules, we can provide different discounts/ different free products for different languages.

Note:
1. Are the products not listed when you type in the discount rules search box ?
For quick access, use the product's SKU id in the chosen language.

Video:

Still unclear ?
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We are always happy to assist you :)