Using French search terms doesn't work on our English website

[sachahcas]

... obviously "Housse Planche" (board bag in French) doesn't produce any results, but we do sell Board Bags. So how can I manually add keywords from other languages to the product page?

Other solutions are very welcome.

Website:
https://eu.nspsurfboards.com 

[shopifyavi]

Just clarifying. Do you already have "Housse Planche" translation in your store or in your translations?

[sachahcas]

I didn't @shopifyavi . I have now added the "Housse Planche" keyword to all relevant product descriptions and hid it in the background. Juvenile and very nineties, but it seems to work.

Now I am wondering if that type of SEO is penalized still...
Ideas?