Learn Hardcore Japanese: watasi ha douryou to issyo ni nihongo wo rensyuusimasu. - I practice Japanese together with my colleague.

In Japanese, は is the topic marker, so 私 (I) becomes “as for me.” It sets 私 as the theme of the sentence (“As for me, …”). が would mark 私 as the grammatical subject, focusing on “who” is doing the action. Here we’re simply introducing the topic (私), not emphasizing “who.”

What does と mean in 同僚と?

The particle と means “with” when it follows a noun. 同僚と literally means “together with a colleague.” It indicates the person you’re doing something with.

Why is 一緒 followed by に?

一緒 on its own is a noun meaning “together.” To turn it into an adverbial phrase (“together [do something]”), you add に. So 一緒に means “togetherly,” i.e. “together.”

Why is 日本語 marked with を?

Particle を marks the direct object of a transitive verb. Here the action is 練習します (“practice”), and 日本語 (“Japanese language”) is what you’re practicing. Hence 日本語を練習します = “practice Japanese.”

What kind of verb form is 練習します?

練習する is a “suru-verb,” where 練習 (practice) combines with する (to do). 練習します is the polite non-past form (“[I] will practice” or “[I] practice”).

Could I omit 私は and just say 同僚と一緒に日本語を練習します?

Yes. In Japanese, subjects or topics (like 私は) can be dropped if they’re clear from context. If you’ve already said “I” or it’s obvious who you’re talking about, you can start with 同僚と一緒に…

Does the word order have to be exactly this way?

Japanese is relatively flexible, but the basic pattern is:
(Topic) + (Companion) + (Manner/Place) + (Object) + (Verb).
You could swap 同僚と and 一緒に (since both are adverbial), but 日本語を練習します (object + verb) should stay together at the end for naturalness.

Is this sentence formal or casual?

It’s polite but neutral. The use of します (the polite form of する) makes it appropriate for coworkers, teachers, or people you’re not extremely close with. In casual speech among friends you might say 練習する instead.