Probably very out of date and not maintained as I don't really use mastodon anymore Feel free to fork or PR!
You're probably better off looking into https://github.com/Gottox/greasemonkey-mastodon-translate as it uses DeepL (which I think is way better than GoogleTranslate) and is actually seeing development.
Just incase you want to still see this one...
You'll first want Tampermonkey/Greasemonkey for your relevant browser then to install this script head to: https://github.com/tomouchuu/mastodon-translate/raw/master/index.user.js and tampermonkey should prompt you to install the script.
Next head to your instance and head to your preferences (/settings/preferences) and there should be an area for the Translation script. If you're happy for the translations to be done via Google then click the checkbox to agree to this and then set the language you want toots to be translated into. Once that's set, click save changes and then head back to your feeds, give the page another refresh to activate the script again (see limitation below).
Upon clicking the ellipsis (...) you should see an option to translate toot, click this and it will translate the toot via a GoogleTranslate proxy (https://github.com/uchuuio/translate-server || https://translate.uchuu.io) into english by default. To change this to your desired language, head to your mastodon preferences in settings and you should see an option for translate language. Select the language you'd like and then save changes, head back to the feed and refresh the page.
If you haven't yet agreed to using Google Translate it will take you to your preferences page where you can enable this option.
If you're having issues with the script, please feel free to reach out to me by raising an issue here or messaging me on mastodon at niu.moe/@tomo or @tomouchuu on twitter.