When you have content in two or more languages which needs to be aligned with the original source it is best practice to have your translation supplier conduct a multilingual text alignment, with the assistance of a professional translator.
The basic process of multilingual text alignment uses translation software to determine which source and target language segments belong together. The software puts the files side-by-side, and attempts to best pair up the source and target text before a professional translator approves and finalises the aligned translation.
This content could come from previous translations, old Translation Memories, inherited content, etc. either way it is content which you could benefit from aligning and using to update your Translation Memory for future re-use.
Once aligned, the updated and approved text can be used to either create a new Translation Memory or update an existing one. Moving forward new translation projects will be analysed against the new or updated Translation Memories in order to leverage content from the aligned text.