I believe the textdomain is being loaded to late, after the acf-pro.php is included.
Moving the load_textdomain function from init() to initialize() solves this issue.
Here is my screenshot for Portuguese translation of an Options Page:
I don’t know why, but from my testing I’ve found that this specific string gets translated if we place a copy of acf-xx_XX.mo file in /wp-content/languages/plugins/ folder, as regular free plugins that get their language packs from translate.wp.org.
Same is happening to me.
‘Update’ button is not getting translated.