Merging some latex documents

Hi, I'd like to merge my job application package in English and German into a single project that I can compile at once, and where I only need to switch a parameter in the top to change languages.

My package consists of 6 tex files and a few PDFs. So far I compiled each tex file separately, as there are different styles (cover letter in a letter style, CV in a CV style, other documents in an article style). The goal would be that they are included in a single file that I can compile at once. The style does not have to be exactly the same as is, but should be similar.

Also, I have separate sources for English and German applications. The goal would be to merge them, such that using a single parameter in the header I can switch languages. In the body I was thinking of something like the following to switch the language of any text: \ifthenelse{\equal{\mylanguage}{german}}{German text}{English text}

Attached are the old application materials (English and German), and what I have in mind for the new main file.