understanding App ranking

[K0RTX]

Hi everyone,

I am a french app creator and I'm trying to understand how ranking in the play store is working.
I thought that ranks were the same for all the countries, but after checking them on appbrain, I saw a big difference.
For example my app is 1st in its category in France, but is not seen in USA.
I'm a bit lost.

my other question is : how to optimize ranking in a particular country ? (without paying any adwords or so campaign)

 

[K0RTX]

I add that I set en-US as default language and that app description was translated in english by an american friend.
I think all the keywords are good, but is there a relative weight between those keywords ?