This feature allows the client to choose the listings they wish to see based on the language used when they were placed. If you had both French and English in use on your site browsing clients could choose to see only listing in English.

Highlights of the language filter:

• The languages id a client was using when they placed a listing is saved with that listings data
• When a client comes to your site they can view the site in the languages you allow them through language choice links or the language choice feature
• If this feature is in use the browsing client will only see listings placed using the language the listing was placed using that match the current language they are browsing the site in. If English was selected when a listing was placed then English is "saved" with that listing and that listing will only appear to clients browsing the site with the English selected.
• note this feature can't account for the client that has English selected on the site yet types all their listing data in Spanish. This feature does attempt to verify the language typed in for a listing….just what language was used when placing the listing. So if English is selected by a seller and they insert a listing with Spanish text that listing will appear to those that select English on the site and NOT to those that select Spanish.

There's only one switch to turn this feature on here in the admin tool:

SITE SETUP > BROWSING SETTINGS > Filter Listings by Language