Is it possible to write articles on dev.to in other languages?

Technically there are no limitations to use another language than English to write articles here.

If I were to write articles in a language other than English, I’d also add a hashtag with the language to it, so you’ll over time receive a nice collection of articles in this language.

I've seen some Japanese articles! If you go to your misc settings there is a feature (in beta) to choose which languages you would like to see in your feed.

Per Terms of Use:

1. Content Policy The following policy applies to comments, articles, and all other works shared on the DEV platform:

• Users must make a good-faith effort to share content that is on-topic, of high-quality, and is not designed primarily for the purposes of promotion or creating backlinks.
• Posts must contain substantial content — they may not merely reference an external link that contains the full post.
• If a post contains affiliate links, that fact must be clearly disclosed. For instance, with language such as: “This post includes affiliate links; I may receive compensation if you purchase products or services from the different links provided in this article.” DEV reserves the right to remove any content that it deems to be in violation of this policy at its sole discretion. Additionally, DEV reserves the right to restrict any user’s ability to participate on the platform at its sole discretion.

So I don't see why not. At worse it will just limit how many interactions your post gets. But if its what you are comfortable with, do it!

I know many Spanish-speaking developers who write articles in Spanish on DEV. And every now and then I see articles in French or Portuguese. As far as I know, there aren't any language restrictions.