复合键：

复合键是由多个列组成的，唯一的行在表中。
复合键可以是主键。

PRIMARY键

PRIMARY KEY约束唯一标识数据库表中的每个记录。

因此它取决于您的需求

第一个设计

 多对多表：
使用composite：
 CountryID Pkey 
 LanguageID Pkey 
 

如果使用此设置，CountryID和LanguageID是复合主key.ie此处

表格数据将为

  CountryId LaguageID 
 1 1 // valid 
 1 2 // valid 
 1 3 // valid 
 1 1 //无效的数据作为其形式复合主键
 $ p> 

和第二个设计 
 使用唯一ID：
 AutoID Pkey 
 CountryID 
 LanguageID 
 
 AutoID成为主键，因此这将允许数据lke thsi 
  AutoId CountryId LaguageID 
 1 1 1 // valid 
 2 1 2 // valid 
 3 1 3 // valid 
 4 1 1 // valid as AutoID is primary key 
 1 2 3 / / invalid as AutoID is prinary key 
 
希望此演示文稿帮助您理解差异
 
I am fairly new to database design, for many to many relationship, what is the differences and implications of creating a composite key and a unique id for e.g.

Country table
CountryID
CountryName
Language table
LanguageID
LangugageName
Many to Many table - using composite:
CountryID  Pkey
LanguageID Pkey
OR

Using unique Id:
AutoID Pkey
CountryID 
LanguageID 

Composite Key : 

A composite key is a combination of more than one column to identify a unique row in a table.
composite key can be a primary key .

PRIMARY KEY

The PRIMARY KEY constraint uniquely identifies each record in a database table.

so its all depend on your requirement 

in first design
Many to Many Table:
Using composite:
CountryID  Pkey
LanguageID Pkey
if you use this desing than CountryID  and LanguageID  is composite primary key.i.e here 

data of the table will be 
CountryId  LaguageID
  1           1 //valid
  1           2 //valid
  1           3 //valid
  1           1//not valid data as its form composite primary key
and in second design 
Using Unique Id:
AutoID Pkey
CountryID 
LanguageID 
AutoID  is become primary key so this will allow data lke thsi 
AutoId    CountryId  LaguageID
 1          1           1 //valid
 2          1           2 //valid
 3          1           3 //valid
 4          1           1 //valid as AutoID is primary key
 1          2           3 // invalid as AutoID is prinary key
hope this presentation help you to understand difference