SELECT 
  cscart_languages.* 
FROM 
  cscart_languages 
  LEFT JOIN cscart_storefronts_languages AS storefronts_languages ON storefronts_languages.language_id = cscart_languages.lang_id 
WHERE 
  1 
  AND (
    storefronts_languages.storefront_id = 1 
    OR storefronts_languages.storefront_id IS NULL
  )

Query time 0.00016

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE storefronts_languages system PRIMARY,idx_storefront_id 0 const row not found
1 SIMPLE cscart_languages ALL 6

Result

lang_id lang_code name status country_code
1 en English A US
2 ru Русский A RU
3 uk Українська A UA
4 es Español D ES
5 fr Français D FR
6 de Deutsch D DE