Resolved in: ACFML 1.3
The 5.8 version of the Advanced Custom Fields plugin introduced a convenient interface to create Gutenberg Blocks.
It uses server-side rendered blocks, where all data is held in block attributes. This includes any text strings used in the block.
WordPress’ Gutenberg hasn’t yet added the ability to filter those attributes and replace them if needed. It means it is not possible to translate text strings kept as block attributes.
We proposed a Gutenberg patch resolving this problem and it is targeted to be included in WordPress 5.0.1 release.
This means that currently, you should either not use WordPress 5.0 yet, or should expect that blocks created with ACF 5.8 will not be translatable.
Update (May 6th, 2019): The required changes are in WordPress now and we are going to release the new version of ACFML with support for ACF Gutenberg Blocks in the beiginning of the May. The target version is ACFML 1.3.