Assuming the parent poster is French because of the reference to France, these pluralization mistakes aren't so far fetched for a Francophone. In French, verbs are conjugated to match whether or not the subject is plural.
How is that different from English? English has subject-verb agreement too.
So, he/she has the word "they" so naturally because this is a plural word, to him/her, "suck" should be pluralized to match.
The plural form of "suck" isn't "sucks"--you don't form the plural form of a verb by adding "s" to it, in either English or French.
Infinitive: "to suck". "He sucks": singular subject, verb ends in "s". "They suck": plural subject, no "s".
Infinitive: "sucer". "Il suce": singular subject, no "s". "Ils sucent": plural subject, no "s".