Hi Andrey,
I have added 4 builds and changed DialogBitmap and BannerBitmap in each build. So i have "en", "ru", "cs", "es", "it" builds.
But when I run my installer on any language I always see the banners from "en" build. 
How can I force it to select build according current system language?
Since a build is defined for each language, all you have to do is to run the related build.
However, there is another approach for this.
Please note that the 
DialogBitmap and 
BannerBitmap already have a property assigned. If you select the 
DialogBitmap in the 
Dialogs page, you can see that its 
Indirect Property is 
DialogBitmap. For the 
BannerBitmap resource is assigned the 
BannerBitmap Indirect Property.
You can set this properties to load a specific resource (e.g. specific bitmap or dialog bitmap).
In order to achieve that, you can go in the 
Images tab from the 
Themes page. For example, you can add a new image for the banner and another one for the dialog for each language.
Then, in the 
Custom Actions page, you can add the predefined 
Set installer property custom action with sequence. You can schedule  this custom action before the 
Searches stage from the 
Wizard Dialogs Stage.
You can add two custom actions, one for the 
DialogBitmap and one for the 
BannerBitmap. 
Also, you need to add this set of custom actions (one for the bitmap and one for the banner) of each language (e.g. French, German etc.)
Since each bitmap and banner depends the installation language, you can condition the custom action execution using the 
ProductLanguage property. (e.g. for French you can add 
ProductLanguage = 1036 statement in the condition field of the custom action).
Please find the attached zip archive which contains a sample project which implement your scenario.
Should there be any difficulty you encounter implementing something, please do not hesitate to contact me and I will gladly assist.
Best regards, 
Dan