There is no such regulation. We're a B2B French company, based in Paris, and there's only one user interface language (English) that even our French customers have to use. Even for B2C, there are no language regulations.

For public and almost-public organizations, there is a regulation: you can communicate in exactly one language (French) or at least three languages (usually these will be English and German).

On the other hand, trying to sell to the average French person will require a French translation, and the same is probably true for most other EU countries if you wish to improve your chances ("have a translation in that language" is likely in the top 5 of improvement suggestions for a new market). 300 million people sharing the same language is a boon, especially for B2C or B2SMB.

Those 300 people also share similar culture, likely speeding up adoption through word of mouth.

Since I mostly consume english, US-centric media, I wonder how high the marketing effect even outside the US is for services that launch in the US first.

As others say, there's no such regulation.

In fact it's better to sell outside the EU as those customers don't have to pay VAT so you have better margins or can sell it cheaper.