If you click on the ‘about’ (the link next to the ‘simple:press’ in the footer of the forum) then you will see that the translation is working just fine. What you are missing is that the vast majority of the text you see displayed is generated by the Simple:Press theme NOT the core file.

Go to the forum admin > integration > Languages and see if there is a translation for the theme you are using. By the way – you do not need the PO file at all.

On the search page then that suggests a rewrite rules error of some sort. Or – perhaps – a mod_secutiry issue on your server. Clearly something does not like the URL so – any security plugins active? Is mod_security being imposed by your host?

Great! I got the translation on now, so there’s a translation file for every theme?

The search is another issue. I don’t have my tech staff available right now, so I can’t verify this. But how would I go about making mod_security changes specific to only this issue? I wouldn’t disable it altogether.

Is there something else I should look for?

I have currently no security plugins active.

HA! It was bbPress. I had it active and deactivating caused the search to get working. 

PS. What would be my best shot for a simple theme with mobile support? 

Glad you got it worked out!

As for a theme with mobile support, we offer two – Unified and Reboot. You can run both as responsive (for mobile) as standard, or you can use a different theme for desktop and enable Unified or Reboot as the dedicated mobile theme in Forum > Themes > Mobile Phone Theme.