The manual for Qt Linguist says:

Changing the Target Locale

If the target language and country are not explicitly set, Qt Linguist deducts them from the translation source file name. For example, app_de.ts sets the target language to German, and app_de_ch.ts sets the target language to German and the target country to Switzerland. This also helps loading translations for the current locale automatically. For more information, see Developers.

If your files do not follow this convention, you can also set the locale information explicitly by selecting Edit > Translation File Settings.

I can't find what is the convention for the translation source file names.

I suggest to add this information.