Invalid Language Marker
This can occur if the second character of a token is a number, making Q++ think that the first character is a LanguageTag. For example, in the token below :
the Z and the 4 were mistyped (the token should have been [4Zd=1]), and therefore Q++ assumes the character before 4 to be the LanguageTag. But as LanguageTags can only range from a..z (lowercase) it emits this error message.
Note that in the above example, if the incorrect token had been [z4d=1], the present message would not have been generated, however there would have been a design-time message that the template was using one more language (based on the LanguageTag z) than were defined.
Another possible cause for this message would be mistyping a DayValue Marker token such as [*1] instead of [1*].