Use any Greasemonkey-like extension you like.
No warranty; use at your own risk.
Duckduckgo quick bang [install]
Add !s and !g to DDG search page.
Twitter Google Translation [install]
Deprecated: Twitter has switched to Google Translate.
If you have other suggestions for alternative translation engine, please create an issue.
A simple and dirty user-script that allows you to translate tweets with google.
Open the user settings page after installation. The script will read your language settings.