If you say ANGLA without LA, it means "a english (thing)" 'cause ANGLA is an adjective. In Eo you say LA ANGLA to mean something like "the English thing", which by context sould be understood like "English language"
LA ANGLA could mean any specificEnglish thing according to context. If I am talking about teas I could say: LA ANGLA ESTAS BONGUSTA.
I think what they are pointing out is that the word "language" could be implied in English as well. After all, "-ish" is an adjectival ending. Similarly, you could say "Siamese are cats" or "Siamese cats are cats," if someone had been talking about a Siamese as a kind of dog.
In English we do not use the definite article there. We could say "English is a language." or "The English language is a language.". according to more than just me. http://grammar.about.com/od/e/g/englishlanguageterm.htm
For one thing, "la angla" is short for "la angla lingvo."
Also, it might have something to do with the fact that Esperanto likes to pull vocabulary from Romance languages. I'm not sure if German does this, but in French you'll sometimes see the definite article appear when you're talking about general nouns. For example, I've seen "English is an easy language" translated as both "L'anglais est une langue facile" and "Anglais est une langue facile."
Using "la" and an adjective without a noun is like saying, in this case, "the English one". We know we're talking about a language, so... Bro, if you don't get it, it's 'cause English. You've got to open your mind to a whole new way of expression. Easier said than done, I know. Call me a jerk, but I was raised bilingual, so yes, I admit I have some advantage... Just remember that not everything is a calque on English. ;)
Different languages have different rules. English was not one of the languages Zamenhof spoke. The rules of Esperanto say that the full phrase is "la L lingvo" but can be shortened to "la L". The rules of English say that we only say "L".
(Where L stands for whatever language we're talking about.)
If "Angla" would be an adjective by itself if you left out "la", then why can't you just add an O to the end, making it Anglo. Wouldn't it be more redundant to say: "La angla estas lingvo" (The english language is a language) rather than: "Anglo estas lingvo" (English is a language)? Or would I be saying "A English is a language"?
"La angla" is idiomatically short for "la angla lingvo". "Angla" is an adjective. "Angla manĝaĵo" is "English food", "angla muziko" is "English music", etc.
"Anglo" is a noun and means specifically "Englishman" (or more broadly, a person from England, just as "franco" means "Frenchman" (or more broadly, a person from France). So saying "Anglo estas lingvo" would mean "An Englishperson is a language".
The accusative is for the direct object of a transitive verb. For example:
La knabino ĵetis la pilkon.
ĵetis is a transitive verb because it performs an action on a thing.
la pilkon is the thing that received the action of the verb.
If you can ask yourself "What got thrown?" -- indeed, if you can re-work it into the passive voice -- then you're dealing with a transitive verb and its direct object.
esti is not a transitive verb. It's not an intransitive verb, either. Transitivity only applies to active verbs, and
esti is part of a class of verbs called stative verbs. They describe the state of something. Anything that comes after a stative verb is called a subject complement. It reflects back on the subject of the sentence.
John looks tired. "tired" describes how John looks (appears).
The coffee smells wonderful. "wonderful" describes how the coffee smells (the aroma it gives off).
La angla estas lingvo. "lingvo" specifies "la angla".
In most languages, this use of the verb "to be" is called a copula. The word that comes after it is not really an object, like a word coming after "to send" or "to hit" would be. Rather, it's usually in the subject form, because the "to be" is really more like an equal sign, so "la angla = lingvo," in a way, and both words are consequently in the same case (grammatical form of the noun).