"Select source branch" empty results when preferred language enabled

Summary

"Select source branch" is empty (No matching results)

Steps to reproduce

1. Go to Merge Requests menu of a project.
2. Try to create a new MR in the "New Merge Request" section.
3. Select a branch from "Select source branch" drop-down

Example Project

https://gitlab.com/TemplateMonster/PlasmaPlatform/Frontend/Plasma-QUARK/merge_requests/new

Current behaviour

The drop-down is empty, as shown on the screen shot attached to this issue. The account settings are default and haven't been changed, neither were the permissions.

Expected behaviour

The "Select source branch" drop-down should display available branches which can be merged into target branch. This works just fine on other accounts for the same project.