Use language locale if the locale file does not exist

When we use locale like a `en-US`, `en-GB`, we get translation missing if `en-us.yml` or `en-gb.yml` is not exist.

before
```
If locale is en-US:
  -> *translation missing* if en-us.yml is not exist.

If locale is en-GB:
  -> *translation missing* if en-gb.yml is not exist.
```

I think this is better.

after
```
If locale is en-US:
  -> *check* en.yml if en-us.yml is not exist.
  -> *translation missing* if en.yml is not exist.

If locale is en-GB:
  -> *check* en.yml if en-gb.yml is not exist.
  -> *translation missing* if en.yml is not exist.