When you write or commission English material, use British English for preference. Tip: set your spellchecker to British English.
There are possible exceptions: for example if a scientist is writing an article for a specialist American publication and wants to serve the reader better.
For translations and translation assignments use clear and businesslike language. As far as possible avoid idioms particular to a country or a culture.
The EUR works with a number of appointed translation agencies with whom tariffs have been agreed. These translators are familiar with our preferred spelling.