Learn Hardcore Japanese: nihongo no rensyuu wo tudukemasu. - I will continue practicing Japanese.

The particle の here marks a noun-to-noun relationship, turning 日本語 (“Japanese language”) into “Japanese‐language practice.” In other words, 日本語の練習 literally means “practice of Japanese.”

Why is 練習 marked with を instead of が or no particle at all?

Because 続けます (the polite form of 続ける) is a transitive verb meaning “to continue (something).” The direct object of that action—the thing you continue—is 練習, so it takes the object marker を.

What kind of verb is 続けます, and how does it relate to 続ける?

続ける is a ru-verb meaning “to continue” (to keep something going). When you make it polite and non-past, you drop る and add ます, yielding 続けます. It expresses either present habit (“I continue…”) or a future intention (“I will continue…”).

Could you use the intransitive 続く instead? For example, 練習が続きます?

Yes, but the nuance changes. 練習が続きます means “the practice continues” (focus on the fact that practice carries on), whereas 練習を続けます means “I continue the practice” (focus on your intentional action).

Why not just say 日本語を続けます without 練習?

続ける needs an activity or object you deliberately continue. 日本語 by itself is a language, not an action, so you need 練習 (practice) or 勉強 (study) to name the activity you’re continuing.

Could you rephrase it as 日本語を練習し続けます?

Absolutely. 練習し続けます combines 練習する (“to practice”) with the continuative –し続ける (“keep doing”), so 日本語を練習し続けます also means “I will keep practicing Japanese.” It’s slightly more verbose but grammatically equivalent.