This is not the first time it happens, I did not find the fix before, I reinstalled/deleted my profile a dozen times until it magically worked.

Now, using latest beta or stable, Debian Sid 64-bit, had it reinstalled since it said it was out of date https://github.com/ValveSoftware/steam-for-linux/issues/747#issuecomment-156189309 but it wasn't.

Steps:

1. start
2. change language to my locale ( yes LC_XX= are all set up; it worked a day before )
3. restart
4. observe it's still English
5. stop Steam

Looking around for another way to change language i see that registry.vdf has such a field, so I change it there:

1. edit file to mylanguage
2. start Steam
3. observe that registry.vdf disappers while steam is loading
4. a new file registry.vdf.old appears and has 0 bytes
5. Steam starts in English
6. stop Steam

Now, registry.vdf is recreated there BUT language is changed to english again.

Editing registry.vdf with mylanguage and changing permissions to 444 (read-only) forces Steam to start in mylanguage