InDesign takes this data from the language settings in the used paragraph or character styles as well as manual overrides. If you only want 1 language to appear in the metadata, you need to set it everywhere the same.
I don't understand
For example i have document with 5 styles set by hebrew
and 1 style set by english (USA)
How should I get only 1 he-IL language determination in content.opf?
If shouldn't, what do these settings mean in content.opf? Smth like such languages are in use?
I have to delete other languages due to any language rather than hebrew appearing there causes problems in reading epub at various devices.
You can't set the language only to hebrew in the content.opf if you used a style with English as language definition. InDesign adds automatically all languages set in the p- and c-styles (This metadata indicates the eReader which languages are contained in the ePub). Its a workaround if you change the language in every p- and c-style to Hebrew, so InDesign will only output Hebrew in the content.opf. But then the epub isn't semantically correct anymore if other languages are used.
Otherwise you need to fix the content.opf manually as mostly only the main language of the publication needs to be indicated there.
So, if only hebrew language appears, despite there are other languages in epub, it is still ok?
You write "indicates the eReader which languages are contained in the ePub" - and if eReader doesn't know i have english word inside, is it a problem?
You need to check this with your customer or publisher. Usually all foreign sentences/words need to be marked with the right language (Language | XHTML Content Documents | EPUB 3 Accessibility Guidelines). The .opf file needs at least one language declaration, but more can be defined if the publication is multilanguage. But its not a must have (EPUB Publications 3.0).
In the case you add other languages via styles to sentences/words, InDesign adds them automatically to the content.opf
This is nothing you can change within InDesign.
The last question whether the order of languages is critical or not? I see that IND cannot put he before en due to alphabet reasons, but if I change it manually?