Learn German: Ich kann Deutsch sprechen.

können (can/to be able to) is a modal verb, and these have irregular patterns. For ich (I) and er/sie/es (he/she/it), they do not take the usual endings, and the main vowel often changes. So, ich kann is simply the irregular form you need to memorize.

Why is sprechen (speak) at the very end of the sentence? In English we say "I can speak German".

This is a classic German grammar rule called the sentence bracket (Satzklammer). When you use a modal verb like kann, it takes the normal second position in the sentence, but it pushes the main action verb (sprechen) all the way to the very end.

Why doesn't sprechen change its ending to match the subject ich?

Because kann is doing all the hard work! When a modal verb takes the second position, the verb that gets pushed to the end of the sentence always stays in its basic dictionary form (the infinitive). Since kann is already conjugated for ich, sprechen does not need to change.

Why is Deutsch capitalized?

In German, all nouns are capitalized. The names of languages like Deutsch (German) and Englisch (English) are treated as nouns, so they always start with a capital letter.