weirdly I get this error when I try to start Mechwarrior 5 using proton 6.8.
I have encountered this issue as well.
i think this issue can be closed, because now it seems like the string is not missing anymore in steam 1647024114

Yep, I can confirm that this is now resolved.
Closing as fixed per "Fixed missing or non-localized text in some dialogs" in the 2022-03-09 Steam client update.
proton 6.8x1 2022-02
Your system information
Please describe your issue in as much detail as possible:
a missing string / translation for
#AppOverlay_UnsavedDataWarningon EnglishSteps for reproducing this issue: