A lot of strings in install|update subsystems are not translatable
1) Maintenance page titles
2) Task lists
3) Installer messages
A first steps, probably this looks like meta issue
A key reason for the update system not using t() is that the locale database can change in updates. Also, using t() direct in the installer could cause DB issues if the page is displayed before the DB is set up. This might or might not be mitigated by the recent changes with locale caching. So I'm not really sure how much if any can we 'fix' of this. Its a matter of the system running its own updates and install that makes us very cautious to run "complex" things like t() that use the database, language list, etc.
related: #1848490: Import translations automatically during installation
This might be possible if/once #1813762: Introduce unified interfaces, use dependency injection for interface translation gets in.
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