Q:

Selecting country in a mobile/tablet landscape mode vs portrait mode on a responsive APP

For selecting country on an APP when it's being opened for the 1st time, On mobile or tablet portrait a 1 column list view is the best option. 
However is it better to switch to a 2 column view when the user flips to landscape, so that all the countries will show without the user having to scroll. tho it would break the consistency with mobile and tablet portrait view. 
p.s. for mobile the app isn't going to changed to landscape

A:

Firstly, ask yourself how many countries the list would need to contain. If the app would only need to show a few (the countries in which markets you sell the app) it may not be worth the effort to apply a different layout on landscape.
Secondly, how likely is it that expats use the app where they actually have to select their country of origin and not the one in which they live. This might already help you to determine the length of the list.
Thirdly, ask yourself whether the app/server does locate the user via IP (good enough on a country-level). If so, the app can show the most likely countries at the very top and all others below. Make sure to add some sort of divider if the list is sorted in some way so that it is clear to the user that the app does make suggestions at the top. 
Lastly, ask yourself how big a bottleneck will this selection process be in context of the overall sign-up flow / onboarding flow. 
Besides, a 2-col list may shorten the length of the list in terms of lines, however, a users' eyes have to quickly jump left-right back and forth in order to find the country name. It is not effortless and does create exise. 

