Most email clients have a provision to create a 'rule'. The rule examines the email as it is received. In generally you can search for specifically senders, recipients,subjects, or specific content words in the body. Based upon the findings of the rule, you can do something with the email (such as end it directly to the Trash).
For example if you know where all of valid you emails come from, you can create a rule that checks the from field for an acceptable sender and sends those email's that to the inbox, while sending all emails from other senders to the trash or spam folder
Keeping out foreign language emails is often difficult because because often it is difficult to come up with a rule that will catch them on the basis of language they are written in.
There are also commercial products that can scan incoming email and 'score' the email based upon content, and you decide at what 'score' you want the mail sent straight to the trash or spam folder. Verizon's spam filter does this based upon Verizon's scoring system, and actually does catch a lot of spam. It also occaisonally catches things that are not spam however....
Hope that is helpful
for foreign language, you are going to have to play detective, and really anazlyze the email and look for common strings of letters or symbols, and then copy those into your rules.