Irule that select pool memeber based on client language in http header

Suggest you try the following...

when HTTP_REQUEST {
  switch -glob [HTTP::header "Accept-Language"] {
    en* { pool pool_app_english }
    jp* { pool pool_app_japanese }
    de* { pool pool_app_german }
    default { pool pool_app_english }
  }
}

The accept language header defined in RFC7231 takes the form of 2 letter abbreviation of language followed by additional information. The above iRule allows you to pick pools for specific languages and even set a default. You can choose not have a default and it will use the pool attached to the virtual server instead.