Usually, abbrevations do end with a dot. And the directly following word after this abbrevation can start with a lowercase letter.
JLanguageTool doesn't detect this if the abbrevation is located at sentence's start. Following German example shows the problem. The abbrevation, "ca.", is at sentence start and the following word, "alle", is marked as wrong, but it isn't at sentence start:
Diese werden etc. eingefügt. etc. und so weiter. ca. alle Leute.
If "ca." is the start of the sentence, it will need to be written with an uppercase letter. I don't see how LT could detect this problem better than now.
Oh, sorry. That's right, but if you change the letter to uppercase, the following word after "Ca." becomes marked as wrong.