Learn Croatian: Učim francuski jezik. - I am learning the French language.

In Croatian, the direct object takes the accusative case. For masculine nouns that are not alive, like jezik (language), the accusative form is exactly the same as the dictionary form. The adjective francuski matches this, so it also keeps its dictionary form. If the object were a living masculine being, the endings would change.

Why is francuski written with a lowercase letter?

In English, words like French or Croatian are always capitalized. However, in Croatian, adjectives made from proper nouns are written with a lowercase letter if they end in -ski, -ški, or -ćki.

Could I say Ja učim francuski jezik instead?

Yes, but the pronoun ja (I) is usually dropped. The -m ending on the verb učim already makes it perfectly clear who is doing the action. You would only use ja here to add emphasis, like saying "I am the one learning French."

How do I know if this means "I learn" or "I am learning"?

Croatian does not have a separate continuous tense like English does. The single present tense verb učim translates to both "I learn" and "I am learning", depending on the context of your conversation.