Learn Finnish: Minä laitan tekstityksen italiaksi. - I put the subtitles into Italian.

The translative ending -ksi is used when expressing a change of state, which includes translating or setting a language. When you read, write, speak, or set a device to a specific language, Finnish uses the translative case to show what language the communication is happening in.

English uses the plural subtitles. Why is tekstityksen in the singular?

In Finnish, tekstitys often refers to the entire track or feature of subtitling rather than individual lines of text on the screen. Because you are selecting the subtitle track as a whole, it is treated as a singular noun.

Why does tekstityksen end in -n instead of being in the partitive case?

It is the direct object of the sentence. Because setting the language is a completed, total action (you are changing the setting completely, not just doing part of it), the object takes the accusative case, which looks exactly like the genitive -n for singular nouns.

Is the verb laittaa the most natural choice here for setting a digital option?

Yes! While laittaa literally means to put or to prepare, it is extremely common in everyday Finnish for turning things on, adjusting settings, or sending messages. You are literally putting the setting into Italian.