Use American English spelling, but mostly British English grammar.
Try to write in a simple, precise and concise manner. Non-native English speakers are likely to be reading the documentation, so it must be easy to understand for as many people as possible.
Open a menu
Press a button
Select a menuitem
Application menu: Open the <app> menu from the top bar and select <menuitem>.
Generic window menu: Press the menu button in the top-right corner of the window and select <menuitem>.