20+ languages sounds like quite a challenge. One way to do this would be to use a combination of Segmentation and Dynamic Content. For the Segmentation, you'd create a segmentation called, say, "Language" and within that you'd have segments for each of the languages. See: Create a Segmentation - Marketo Docs - Product Documentation.
Then within each email, you'd assign a different version of the various copy modules to the appropriate segment: Using Dynamic Content in an Email - Marketo Docs - Product Documentation.
You could also have 20 different emails (one per language) and use a smart campaign to send the right one based on the Segmentation or a Language field.
However, either of those methods would be pretty labor-intensive if every email has to have 20+ different versions. I don't know whether or not there is some way to code the different language choices into the email HTML or not. Somebody else in the community may have a better idea.
Quang, can you move this thread to the central Community discussion area at Products? This is a members-only group and I just happened to see your thread because I follow Denise.
In short the equivalent to the SFMC conditional content in Marketo is a Velocity ("Email Script") token. Velocity is a extraordinarily robust customization language based on Java (and which predates Marketo itself). It can check the lead's language and render content accordingly -- that's one of its core competencies.