What ______________________________ I don't speak another language well?

The phrase "What worries me is that I don't speak another language well" is the most suitable choice because the verb "worries" effectively conveys a sense of concern or anxiety about a specific issue. In this context, the speaker is expressing a feeling of unease regarding their language skills, which fits perfectly with the use of "worries."

The other options, while each present a valid emotion, do not align as well with the nuance implied in the statement. "Concerns" can also denote a certain level of worry but tends to be more formal or impersonal. "Upsets" carries a stronger emotional connotation, suggesting a more intense reaction, while "troubles" tends to imply a problem that may be more persistent or enduring. However, when discussing one's language ability and the feelings it surfaces, "worries" captures the appropriate level of anxiety and is, therefore, the best fit for the statement.