Update default language to English (#309)

### What problem does this PR solve?

Update default language to English

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/db/db_models.py

         max_length=32,
         null=True,
         help_text="English|Chinese",
-        default="Chinese")
+        default="English")
     color_schema = CharField(
         max_length=32,
         null=True,
     language = CharField(
         max_length=32,
         null=True,
-        default="Chinese",
+        default="English",
         help_text="English|Chinese")
     llm_id = CharField(max_length=32, null=False, help_text="default llm ID")
     llm_setting = JSONField(null=False, default={"temperature": 0.1, "top_p": 0.3, "frequency_penalty": 0.7,

web/src/pages/chat/chat-configuration-modal/assistant-setting.tsx

       <Form.Item
         name={'language'}
         label={t('language')}
-        initialValue={'Chinese'}
+        initialValue={'English'}
         tooltip="coming soon"
         style={{ display: 'none' }}
       >