WFB> In other words I want all rows with "LANG_CODE5" = 'FR_fr", plus all
WFB> with "EN_en" where there is no corresponding "FR_fr".

Try the following. But I wrote that together quickly and am not sure
if it works as desired for you:

Select
T1.ID
from
TheTable T1
where
T1.Lang_Code5 = 'FR_fr'

UNION

Select
T2.ID
from
TheTable T2
left outer join
TheTable T3
on T2.Code_ID = T3.Code_ID
and
T3.Lang_Code5 = 'FR_fr'
where
T2.Lang_Code5 = 'EN_en'
and
T3.ID is null