Hello @rlach, this reads like an issue with the Steam client instead of SteamOS as a platform, so I've transferred this issue report to the steam-for-linux issue tracker.
I second that.
I've recently been using workaround for this (which doesn't mean this is obsolete), if someone is struggling with this issue.
Go to desktop version of steam client, which will display guides properly in all languages you need. Add a guide to favourites. The guide will now appear properly on SteamDeck in game mode regardless of language you have set. This still means you have to use other device or switch to desktop mode to find the guide(s), but at least you can use them in game mode without sacrificing your native language.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
In game mode, when language is set to any other language than English the Guides overlay for games will show little to none guides. The reason for that is vast majority of guides is written in English. Even non-native steam users write and use English guides.
If the user opted for secondary languages in the steam language settings guides written in those languages should be visible.
Alternatively SteamOS itself should be able to configure such languages. Or the language filter in Guides overlay should be user-adjustable. Any solution to allow using non-English OS with English guides would be fine.
Steps for reproducing this issue:
For comparison the same game when steamOS is set to English 18 PAGES of guides. That is around 180 guides.