Solution:
If your selected strings' default language is different than site's default language, you'll see a message at end of strings table:
Selected strings are not in the site's default language and will not be translated automatically if you're using the "Translate Everything Automatically" mode. Instead, after sending them for translation here, you need to go to the WPML -> Translations page and translate them manually.
Either you'll have to translate these manually, or, you'll need to change the language of these strings to site's default first and then can send for automatic translation. Because automatic translation only works one way like Default Language to Secondary Language.
To translate strings from String Translation automatically, please go to WPML > String translation, select and send for translation.
If you have enabled Translate everything they will be translated in the background, otherwise go to WPML > Translation, "Take and Translate" translation job, and in Advanced translation editor click "Translate automatically".
