Sorry for the late response,
I cannot provide a precise answer of what to do since it happens on the software side. The translation variable in the code should integrate the colon {:}
As an example, the key theme_label should not have as a source "theme" but "theme:", because if the colon is outside of the key (and are therefore hard coded), it is not possible to translate it (and to add a space before, in accordance with French grammatical rules)
The change must be made in the code, not through the translation tool. Users who had to translate "Theme" must now have to translate "Theme:". In addition, this could in some cases involve a duplication of keys, since some of them must have two versions, one with the colon and one without (this is the case for the start_page key, which appears in the window of Settings with the colon and which appears as tab title without it).
This surely involves identifying and translating all the faulty keys again. This is obviously an important effort, especially since it is a problem that only concerns the French language.
More information on the colon: https://en.wikipedia.org/wiki/Colon_(punctuation)#Spacing_and_parentheses