How to get the size of a Stream after applying a filter by lambda expression?

When you get a stream from the list, it doesn't modify the list. If you want to get the size of the stream after the filtering, you call count() on it.

long sizeAfterFilter = 
    locales.stream().filter(l -> l.getLanguage().equals("en")).count();

If you want to get a new list, just call .collect(toList()) on the resulting stream. If you are not worried about modifying the list in place, you can simply use removeIf on the List.

locales.removeIf(l -> !l.getLanguage().equals("en"));

Note that Arrays.asList returns a fixed-size list so it'll throw an exception but you can wrap it in an ArrayList, or simply collect the content of the filtered stream in a List (resp. ArrayList) using Collectors.toList()(resp. Collectors.toCollection(ArrayList::new)).

Use the count() method:

long matches  = locales.stream()
  .filter(l -> l.getLanguage() == "en")
  .count();

Note that you are comparing Strings using ==. Prefer using .equals(). While == will work when comparing interned Strings, it fails otherwise.

FYI it can be coded using only method references:

long matches = locales.stream()
  .map(Locale::getLanguage)
  .filter("en"::equals)
  .count();