I'm making a plugin, main logic of which is in the Javascript controller (dev/js/myscript.js). Is there any easy way to use language strings in this external Javascript file?

Now I have to manually assign needed strings to Javascript variables in the template like this:

var today = {lang="today"};

But I need a lot of language strings, and it's becoming too hard to use this approach.

The same question with the other template plugins like {setting="$settingKey"}.

Normal: 

ips.getString('key')

With %s: 

ips.getString('key', { parameter1, parametr2 etc } )

With pluralize: 

ips.pluralize( ips.getString( 'key' ), parameter )

Yes, don't assign language strings to variables - in your js file just use the appropriate methods (as @Ilya Hoilik showed) to fetch the strings where needed.

Thank you very much, @Ilya Hoilik and @bfarber!

As I understand this allows us to use only current plugin's language strings, not all. Am I right?

And the second question about the settings. As I see in the code, I can't simply use getSetting(settingKey) without predefining settings using ips.setSetting(settingKey) in the templates. Is it a correct way?

Hello,

You can reference any language string - ultimately javascript language strings are all set in the same manner and are globally accessible.

That is correct about setSetting/getSetting.  Not all ACP settings are passed to javascript inherently, you will need to manually pass any that you want to access.