This article has been written before more than 24months, information might old.
Some time ago I read about a critical vulnerability present in WordPress 4.7.0 and 4.7.1(it was seriously severe, it primarily let attackers change data in your DB using restful API) , and I had a website that I did back in 2015, problem is that I did the theme using _e function(mostly on the front page) for translations as it was a multilingual website.
Anyway, I updated from 4.7.2 to 4.7.3 and <<BAM>> all translations were broken. It was a bit curious though why this update broke the translation if in 4.7.0 they worked fine, so started to see if any change was written in the official documentation that I could find on this links: I18n_for_WordPress_Developers and developer.wordpress.org/themes/functionality/internationalization.
But at the time of running, that update I didn’t see any major change, I made a little debugging and noticed that the Text Domain: was loading fine, as I tried an echo in the front page, but still no translation.
Long story short I found that that is better to have the languages in your new folder from wp-content called languages that way you don’t have any problem but, problem was that the language files weren’t copied there during the update after manually putting them there it worked like a charm.