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So sddm seems to be starting just fine. Seems totally unrelated to your locale issues.
What video drivers are you using?
I installed nvidia-dkms, nvidia-utils and lib32-nvidia-utils. Now, I have to login and go 'sudo sddm' and sddm "works". I login on sddm and then it takes me back to sddm.
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Issue resolved itself.
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#26 indicates that the session crashes for "reasons" - you'd have to look up an older journal for any idea of "why" but it's most likely the nvidia driver installation - the locale issue was resolved by re-installing the filesystem package.
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