The Custom BBCode creation page says

"Within the HTML replacement you can also use any language string present in your language/ directory like this: {L_<STRINGNAME>} where <STRINGNAME> is the name of the translated string you want to add. For example, {L_WROTE} will be displayed as “wrote” or its translation according to user’s locale."

This is not working for "any language string"! If there is an integer in the <STRINGNAME>, it will not be rendered, and instead outputs as {L_STRINGNAME}

For example, create a simple custom BBCode.
Use a common lang var in the HTML replacement code that has a number, i.e.: {L_1_DAY}
That will render out in a post as {L_1_DAY}.

Change that BBCode to use a lang var with no number in it, i.e.: {L_DAY}
That will render as expected displaying the correct string value for the lang var, in this case, "Day"