Keyword clustering is not language-specific but rather geo/country-specific. We support all the countries in the world.
Let's take a look at a few examples.
I have an SEO client in France, and I'm targeting users in France. When I prepare my keyword list, I will select all the keywords and search volume data for those keywords. I will choose France under the country dropdown when I run my order through Keyword Insights.
Keyword Insights will then scrape google.fr and build the clusters based on how keywords are ranking in google.fr.
You can choose to upload French keywords, English keywords, or any other language. Regardless of language, Keyword Insights will scrape google.fr for those keywords.
As you can see, the language has no influence whatsoever on our clustering process.