.strings File

.strings files are used in the translation process for iOS, MacOS, TvOS and WatchOS. The preferred way to automate the iOS app translation with Applanga is by using the Applanga iOS SDK . Alternatively, if you can not use the SDK or if you want to localize a MacOS app, you can also use the Applanga Localization CLI to automate the up and download of the .string files.
A .strings file can have a accompanying .stringsdict files to provide support for pluralization.
You can create and edit base strings directly in XCode and the translated strings can be referenced in your xib and storyboard files.

Website: https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPInternational/LocalizingYourApp/LocalizingYourApp.html

File Extension
.strings
CLI format key
ios_strings
CLI Supported
true
SDK Supported
true
String Description Supported
true
Pluralization Supported
false

Example:

"empty_string_key" = "";
/* Here is the description of the key */
"with_description_key" = "This key has a description in some file-formats.";
"with_line_break_key" = "This key has a line break\nin it.";
"nested_key.level_one_a.level_two" = "A deeply nested key.";
"nested_key.level_one_b" = "A nested key.";
"array_key[0]" = "first entry";
"array_key[1]" = "second entry";
"array_key[2]" = "third entry";
"simple_key" = "Most basic translation.";
"plural_key" = "";