Which language to choose for standard wordpress translaion

Both should work, but the default language of WordPress is en_US, so it probably makes more sense to use that if you’re just want alternative English terms for everyone.

You’ve not said what you’re translating, but this will work with the WordPress core and the twentyfourteen theme. (I’ve not tried others)

If you’re changing the terms in third party plugins and themes, this will only work if they’re set up properly.

Hi Tim, Thans for your reply.
I am trying to change woocommerce – i saw somewhere that loco supports woo. Are you aware of any inconsistency between these two plugins.
I am trying to change the standard phrases – I was dabbling with en-GB. I will try with en-US and let you know.

Loco supports any theme or plugin, but that doesn’t mean you won’t run into issues.

If translations are not showing up, there are dozens of possible reasons.
The most common is the wrong Text Domain, or multiple text domains being used. Some plugins even pull translations from other plugins that may not be installed. These issues are beyond what Loco can do. All it does is manage your PO and MO files.