It seems that language editor can not add new items in its string list. And names of my custom pages are surely not already there.
So, detailed solution is :
- open App_GlobalResources/GlobalResources.resx in text editor;
- find name of some DNN's predefined page, for example search for string "ActivityFeed";
- copy and paste whole <data> tag around ActivityFeed string;
- change it to adhere to your page name; (avoid spaces in "name" attribute)
- "Save as" to make a copy for other language, with appropriate infix (sl-si in my example) in file name;
- change new data tag in new file version, in order to translate it.
That's it. You can reload page and see results.