Learn Hardcore Romanian: Îndată ce compania aeriană localizează valiza, trimite numărul de referință. - As soon as the airline locates the suitcase, it sends the reference number.

It is perfectly correct, but it sounds a little more formal or written than some everyday alternatives. In conversation, many speakers would also say:

imediat ce
de îndată ce (also somewhat formal)

So in this sentence, îndată ce compania aeriană localizează valiza means as soon as the airline locates the suitcase.

Why do compania and valiza end in -a? Are they definite?

Yes. Both are definite singular feminine nouns.

In Romanian, the definite article is usually attached to the end of the noun, unlike English, where you use a separate word such as the.

companie = airline/company
compania = the airline/company
valiză = suitcase
valiza = the suitcase

So:

compania aeriană = the airline
valiza = the suitcase

This is one of the first big differences English speakers notice in Romanian: the article is often suffixed to the noun.

Why is it compania aeriană and not ?