Since it's a decentralized social network, Mastodon is more difficult to search than a centralized platform like Twitter. But that doesn't mean it's not searchable. Every instance is a domain, after all, and domains are searchable via Google's site: search.
Mastodon Web Space Search lets you search for Instances in two ways -- by language or keyword -- and then search those instances using Google. It will only find Instances with more than 200 active users. Mastodon Web Spaces uses the Instances.social API.
1. Enter the search you want to run across the Mastodon instances you choose. The instances will be bundled into a Google query that includes your search, so you can use whatever Google syntax you like.
Now, Choose Your Search Type
You have two ways to find Mastodon instances to search: getting a list of the most active Mastodon instances by language, or by doing a keyword search of instances.
2. Active Instances by Language:
Specify the primary language for the instances you want to find. Use two-letter language codes (en for English, fr for French, etc.)
In my testing I found most languages. If you're getting errors or no results, try another language and/or make sure you're using language codes and not country codes (the country code for Japan is JP, but the language code for Japanese is JA.)
2. Active Instances by Keyword
Enter the keywords for instances you'd like to find. General keywords -- country names, general interest categories -- work well. Examples: Germany, technology, science, engineering, social, news, safe