When your audience is not monolingual.
THE QUESTION FIRST
Is it necessary? Adding a language doubles the content to write and maintain.
If your customers all read English comfortably, it may not be worth it.
IF YOU DO
Translate properly. Machine translation reads badly and damages credibility more than having one language.
Translate the pages that matter rather than everything badly.
STRUCTURE
Separate URLs per language, typically in subdirectories.
A clear language selector, visible on every page.
WHAT NOT TO DO
Redirect automatically based on location, which frustrates visitors and prevents search engines seeing other versions.
MAINTENANCE
Every update must be made in every language, or versions drift apart and some visitors see outdated information.
This is the real cost, and it is ongoing.
THE PRACTICAL ALTERNATIVE
One language done well, with clear simple writing that non-native readers can follow.
For many small businesses that is the better choice.