Nintendo Entertainment System
In Europe and Australia, the system was released to two separate marketing regions. One region consisted of most of mainland Europe (excluding Italy), and distribution there was handled by a number of different companies, with Nintendo responsible for most cartridge releases. Most of this region saw a 1986 release. Mattel handled distribution for the other region, consisting of the United Kingdom, Ireland, Canada, Italy, Australia and New Zealand, starting the following year. Not until the 1990s did Nintendo's newly created European branch direct distribution throughout Europe.

Q: Who released cartridges in mainland Europe?
Nintendo