Do you want the first document to continue to have both languages, or do want to end up with each language in a separate document?
How large is the document?
Is it in paragraphs? Do any do the paragraphs contain both languages?
Here's one possible method of separating the text, using Numbers as a 'helper' application in which to do the processing.
If needed, start by separating the document into paragraphs, each containing only one of the languages. It's OK to have two or more consecutive paragraphs in the same language; what is important is to have a return character where the language changes.
Click anywhere in the text, press command-A to Select All, then Copy.
Your table should now resemble the one below, with one major exception: Where mine shows a single word, your cells will each contain a full paragraph in one of the two languages. If there are any places in your original document where you have pressed the return key twice, you will see an empty row. Ignore these, as they'll be cleared later.

Make these adjustment to the table:
Delete columns E, F and G (and any others to the right of these—you'll need only 4 columns).
In the row of column reference tabs, click and drag to select columns B, C and D. These three tabs will change to blue. Place the mouse pointer on the line separating B and C. When the pointer changes to a vertical line crossed by a two-headed arrow, click and drag to the right to make all three columns wider (and the table shorter, as the text wraps into fewer lines.
Click on cell C2, type = to open the formula editor, and enter the following formula:
IF(LEN(B2)<1,"xxx",IF(A2,B2,"xxx")
Click the green check to confirm the formula.
Click on cell D2, type =, and enter the following formula:
IF(LEN(B2)<1,"xxx",IF(A2,"xxx",B2)
Click the green check to confirm the formula.
Check the operation of the formulas by clicking the checkbox in cell A2 a few times.
With the box checked, the contents of B2 will be copied into C2; with the box unchecked, the contents of B2 will be copied into D2,
If the formulas do not behave as expected, check the examples, and edit the formulas to match.
When you are satisfied that the formulas are working as expected, select cells C2 and D2, the use the mouse to grab the Fill Handle (yellow circle) that appears on the bottom edge of the selected pair, and drag down to fill the formulas into the rest of columns C and D.
With the exceptions noted above, your table should now look like this:

Separating the languages:
Examine each paragraph in column B. Click to check the checkbox in column A beside each row that contains Language 1. Leave unchecked each box in a row containing Language 2.
When done, the table will look like this:

Check columns C and D to ensure that each contains only one of the languages (plus several xxx rows). Click the checkbox for any rows where the language is in the wrong column.
With all the paragraphs now separated into two columns, there are two tasks to complete for each language before pasting it into new Pages document:
- Copy only the cells containing text in that language.
- Remove the formatting that will cause the copied text to paste as a single column table.
To do number 1, you will need to filter out all the rows with xxx in the column being copied.
Click any cell on the table.
Click Sort and Filter (top right corner), then Filter.
Click Add a Filter.
Choose Column C
Choose Text, is not...
Enter xxx in the box
Click to check the checkbox beside Filter to apply the filter.
Double click the column reference tab for column C. Copy.
Click the T box in the row of buttons above the workspace to insert a text box.
Drag the box to an empty space on the sheet, click inside the box, and Paste.
You now have a text box containing all the paragraphs in Language 1.
Select and Copy the text in one of the text boxes.
Open a new Pages document, click in the body space, Paste.
Back in Numbers:
Click on the table to reactivate it.
Click the Filter checkbox to deactivate the filter and show all rows.
Repeat the steps above for Column D.
If you will be doing this again, clear column B of the Numbers document, then Save the document.
Regards,
Barry
Click in the text, press command-A to Select All. Copy.
(This step changes the format of the copied cells from a single column table to a string of text in paragraphs.)
Open a new Pages document. Click in the body space. Paste.
To remove the extra returns:
Press command-F to open the Find/Replace dialogue.