Notice: Backup your site and database first before applying this update!
From version 3.1.2 OJS provides multilinugal naming for authors and users. So there is an issue to get back to official OJS repositories and save accumulated information.
Needless to say that from version 3.1.2 OJS moved to new naming scheme: from first name, last name and middle name (three fields) to given name and given family (two fields). So for western authors given name field should contain first name and middle name, separated by a space.
To update OJS from my branches for versions 3.1.1 to stable branches with 3.1.2 you should convert your database with commands on the link below.
SQL commands for database conversion https://github.com/litvinovg/ojs/blob/3.1.2-multilingual/multilingual-3.1.1_to_3.1.2.sql
If you only used authors multilingual support in my branches, then from now you can switch to official OJS branches for version 3.1.2, there shouldn't be any problems.
If you still need citations multilingual support (which is not supported in 3.1.2), then you can use my repositories for 3.1.2 listed below. These changes are small, so you can save them as patches for your OJS.
That is my OJS branches for 3.1.2 :
https://github.com/litvinovg/ojs/tree/3.1.2-multilingual
https://github.com/litvinovg/pkp-lib/tree/3.1.2-multilingual
https://github.com/litvinovg/citationStyleLanguage/tree/ojs-3.1.2