The wife of an insurance magnate, suspicious that her husband is having an affair, sends him a lascvicious invitation from an ''anonymous'' admirer, just to see if he'll show. The action of ''A Flea in Her Ear,'' one of the best-known French farces, takes us to the sleazy Lanterne Rouge Hotel with a fine crop of mocking, mistaking, insulting, groping, kicking and mimicing, all in a spanking new translation.