Hey PeterPan,
*1 Can you be more specific on the language question? What theme are you using, what translations have you downloaded i.e core, theme, plugin translations, and of course what language?
Also, what pages are you finding untranslated?
If you’re interested you can become a translator to create translations and also a validator to approve them.
*2 Sorry not sure I understand. Are you asking how to hide the footer and stats to members but display to moderators and admins? Or how to hide moderators and admins from the stats section?
*3 Depending on what you mean by forum images, if you mean icons you can do this through the Custom Icons and Manage Groups and Forums options in the admin menu, start by uploading your icons in Forum > Forums > Custom Icons, then select them in Forum > Forums > Manage Groups and Forums > Edit Forum or Edit Group.
However, if you’re referring to button and other images, they can be found within the theme. If this is what you want to change you will want to make a child theme, then use your own images with the same file name in your child theme folder, and they’ll be loaded instead.
*4 It’s really the same for Simple:Press as it is for WordPress, in general the more plugins you have activated the slower things will run, although chances are you probably wont notice a huge difference. That one you will have to try and see!