Translation:English is the international language.
You can always say "Η Αγγλική (γλώσσα)". But it's more common colloquially to say τα αγγλικά. It's probably a nounified adverb Μιλάει Αγγλικά=He speaks Englishly (in an a english way), and then αγγλικά became a plural noun because it looks like it. Or it is a nounified adjective from "αγγλικά λόγια"="english words" with λόγια omitted over the years.
The adjective is διεθνής, not διεθνός.
It's not a three-ending adjective in -ος, -η, -ο or -ος, -α, -ο; instead, it's a two-ending adjective in -ης, -ης, -ες.
Masculine and feminine endings are identical (thus there are only two distinct sets of endings, not three) and the inflection is a bit different from the three-ending adjectives in -ος, -η, -ο.
There are also a few other kinds of endings for two-ending adjectives; for example, there is also -ων, -ων, -ον as in ευγνώμων "grateful", or -ος, -ος, -ο as in έγκυος "pregnant" where the feminine form ends in -ος as well.
Three-ending adjectives in -ος, -η, -ο are the most common, but not the only kind.
(Another kind of three-ending adjective is -ων, -ουσα, -ον as in ενδιαφέρων "interesting".)
In case you're curious, this ων, ουσα, ον class includes what used to be Ancient Greek active present participles and now only survive in fossilised adjectives like this one -- ενδιαφέρων is literally "interesting" in that it's the present participle of the verb ενδιαφέρω "to interest (someone)" just as "interest-ing" in the present participle of "to interest".
(The modern present participle is in -οντας, is invariable, and only acts like an adverb, as in Μπήκε τραγουδόντας "he came in singing" = he came in while he was singing.)