结果

[
  {
    "language_code": "en",
    "value": {
      "Cancel": "Cancel",
      "User address": "User address"
    }
  },
  {
    "language_code": "zh",
    "value": {
      "Cancel": "取消",
      "User address": "使用地址"
    }
  }
]

SQLite 实现

WITH
  data(lang, key, value) AS (
    VALUES
      ('en', 'Cancel', 'Cancel'),
      ('en', 'User address', 'User address'),
      ('zh', 'Cancel', '取消'),
      ('zh', 'User address', '使用地址')
  ),

  languages(lang) AS (
    SELECT DISTINCT lang
      FROM data
  )

SELECT json_group_array(
         json_object(
           'language_code', lang,
           'value', (
              SELECT json_group_object(key, value)
                FROM data
               WHERE lang = t.lang
           )
         )
       )
  FROM languages t;

可以先把所有符合的数据取出来，然后利用laravel封装的集合方法，groupBy()多级分组，去实现。这个多级分组还是挺有效的。