Or convert all files in the directory:

tikal -2tmx -fc okf_xliff-sdl *.sdlxliff -sl de-DE -tl ro-RO

At least version M35 is needed.
From the Tikal WIKI:
Convert to TMX Format

Creates a TMX document for the give input file. If the input file is multilingual (like a PO or a TS file), the source and target will be in the TMX document.

The syntax of this command is:

-2tmx [options] inputFile [inputFile2…]

Where the options are:
-fc configId The identifier of the filter configuration to use for the input files.
-ie encoding The encoding name of the input files. this is used only if the filter cannot detect the encoding from the input file itself.
-sl srcLang The code of the source language of the input files. See more details…
-tl trgLang The code of the target language. See more details…
-trgsource|
-trgempty Forces the content of the output target field to be either a copy of the source or empty. If neither option is set the content of the target field is the target text or empty.
-all Allows entries that have no text to be converted. If this option is not set (the default), the entries that are empty, or contains only codes or whitespaces are not included in the output file. If this option is set all entries are included in the output.
-approved If this option is set only approved entries are included.
-rd rootDirectory The root directory (by default the user’s home directory).

For example:

tikal -2tmx data.po -sl EN -tl ZU

Creates a TMX document from the PO file data.po. The source language is English and the target Zulu.

tikal -2tmx data.tmx -sl EN -tl DE -trgempty

Creates a TMX document from another TMX document named data.tmx. The source language is English and the target German. The content of the elements for the German will be empty.