Sotte que je suis, j’ai étudié le français!

The possibility considered above has the defect of being broken up into two parts each with its conjugated verb and that makes for a degree of connection less immediate, not so directly perceived as when there is just one conjugated verb as in the next possibility; this one is a variant that has only one conjugated verb and that is an utterance in a single part, unbroken by a comma; the connection by the preposition « de » is explicit.

Sotte que je suis d’avoir étudié le français!

Moi, parlant pour moi et de moi : Imbécile ! T’as étudié le français ! (Oui je me tutoie… en privé…)

Plus rarement, parce que la conscience de soi… ça fait plus mal : Qu’est-ce que je peux être c** ! J’ai étudié…

Si je devais écrire cela dans un livre… Hmmm… non mais quelle idée ? Avouer cela publiquement… 😉 : Mais suis-je bête !, j’ai étudié…

I understand that she feels bad about electing French and not German in her schooling, as German would have proven more useful, if only she could have evaluated the future a little better.

Mentionning explicitely the choice that failed to be made is a common way to express this in French:

• Quelle idiote d’avoir choisi d’étudier le français !
     Word for word: What a fool to have elected to study French!
     What it means in more coloquial English: Why did I choose to study French?
• Dire que j’ai étudié le français, quelle nulle !
     What it means in more coloquial English: …and I studied French! Such a silly girl!

Nota bene:

1° Unlike in English, names of languages in French start with a lower case letter (anglais, français, islandais, japonais, russe, etc.).

2° A non-breaking space is used in French in front of punctuations that use two signs (as the colon, semi-colon, or in this case the exclamation mark). I don’t think anyone will pay too close attention to that detail when the main text is entirely in English, but you have a choice here to make it that bit more exotic by enforcing the French rule for the French sentence.

3° Other choices are possible, of course, and the votes and comments should (hopefully) ultimately allow the best choices to move ahead of the less satisfying ones. These last few comments will however apply to any past and future proposition.

For my part, I would use something a bit more verbose. It reads a bit sarcastic, but I believe this would fit to your need.

• Zut alors, l’idiote que je suis a appris le français !