You can edit the language variables on the Administrations > Languages > Translations page in the admin panel of your store. Select your second language on this page and use the search in the sidebar to find the desired language variables.
The language variables used in the mobile storefront are starting with twg_ and the Twigmo language variables that are used in the admin panel are starting with twgadmin_ .
Also, you can override translations used in the desktop version. For example, the "add_to_cart" language variable has the "Add to cart" value. You can create a new language variable with the "twg_" prefix: "twg_add_to_cart" and with the "Buy" value. As a result in the mobile version the "Buy" value will be displayed instead of "Add to cart".