The good news: One intrepid blogger offers up a massive cut-and-paste Gmail filter operator that corrals and archives non-English messages. The bad news: It only works when spammers are semi-honest.
No matter how devious the realms of herbal solutions and DEAR SIR notes, Digital Inspiration's filter is probably worth installing in your Gmail filters. Create a new filter by clicking Settings->Filter->Create a new filter in Gmail's web interface, then follow the linked post's instructions on pasting and setting up a system to archive and, optionally, apply a label to your non-English mail. For those of us who have no contacts who would write in another language, it's fairly safe to just delete or archive the filter-caught messages without a new label, but international mailers can use their discretion.
The bad news is that this filter will only catch emails that identify themselves in their headers, or embedded source information, as written in a language other than English. Spam mail has about the least reliable headers around, which is why thieves write messages from "PayPal Services" asking for your passwords. So don't expect to see everything cleaned away, but with a combination of this filter and Gmail's built-in spam management, you might get a cleaner inbox. For more plug-and-play filter fun, try our ten must-have, download-ready Gmail filters.
Reduce Spam in Gmail Inbox with a Language Filter - Block Foreign Languages Emails [Digital Inspiration]