If you only want to localize your website into your main language, we recommend you use the free plugin Loco Translate. Make sure to read our tutorial on how to translate Merchant with Loco Translate.
If you want to have a multilingual site, i.e. one that is translated into more than one language, we recommend using either Polylang or WPML. We have tutorials on how to use either plugin to do this – for Polylang here and for WPML here.
If, with a multilingual site, you want to translate strings that are stored in the database, i.e. strings that are defined under the module settings, you must use either Polylang or WPML. For a single language site, you would simply write the string you want in the language of your choice in the module settings.
If you want to translate strings that are hardcoded into Merchant, i.e. where there is no option to define the text from the module settings, you must use Loco Translate.
A note on how language switching works with Polylang/WPML. When you switch to another language, WPML/Polylang will automatically update the WP_LANG to set it to the selected language and then load everything related to that language, e.g .mo, .pot files for the respective language selected as well as load the translated strings from the database for that selected language.