Hi,
I’m using WPML – http://wpml.org/ – on a complex project and everything translates great. It has a logical interface for translating anything – I’m translating a lot of ACF fields without problems (and of course all the regular WP stuff).
I’ve done a lot of research and WPML always came recommended as the most reliable solution. It does add some overhead, but once I cache everything properly, it’s not a problem.
If I would make only a two-language website and the editors could all handle english captions for backend, I would rather create duplicate fields for everything. But in this case, the site would have to be an exact mirror between languages.
If you want the ability to only translate certain things and have multiple languages, I think WPML is your best bet. It costs a bit, but it’s a multi-site license. There might be something faster out there, but I’d be worried about support – once something changes, are they big enough to update everything quickly? WPML is so widely used, I’m sure they are under enough pressure to provide up-to-date updates for all the major plugins, ACF included.
If you do find something lighter and faster, let me know!
