Learn Russian: Русский язык очень красивый.

In Russian, the present tense of the verb "to be" (am, is, are) is almost always left out. Instead of saying "is very beautiful," you just put the words together as "очень красивый" (very beautiful), and the "is" is naturally understood by the listener.

Both русский and красивый are masculine adjectives. Why does one end in -ий and the other in -ый?

Masculine adjectives usually end in -ый, like красивый. However, Russian has a famous spelling rule that says you can never write the letter ы after к (along with six other specific letters). Because the base of русский ends in к, we have to swap the ы for an и, making it -ий instead.

If the word русский was in the middle of a sentence, would it still be capitalized?

No. Unlike English, Russian does not capitalize the names of languages or adjectives of nationality. It is only capitalized here because it is the very first word of the sentence.

Does the word язык only mean "language"?

язык translates to both "language" and the physical "tongue" in your mouth. Just like in English when we say "mother tongue," the two concepts share the same word in Russian. You will always be able to tell which one is meant based on the context.