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SELECT 
  IF(
    cscart_ult_language_values.value IS NULL, 
    lang.value, cscart_ult_language_values.value
  ) as value 
FROM 
  cscart_language_values lang 
  LEFT JOIN cscart_ult_language_values ON cscart_ult_language_values.name = lang.name 
  AND company_id = 1 
  AND cscart_ult_language_values.lang_code = lang.lang_code 
WHERE 
  lang.lang_code = 'en' 
  AND lang.name = 'important'

Query time 0.00025

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	lang	const	PRIMARY	PRIMARY	392	const,const	1
1	SIMPLE	cscart_ult_language_values	const	PRIMARY	PRIMARY	396	const,const,const	0	Unique row not found

Result

value
Important